if not modules then modules = { } end modules ['font-otl'] = {
    version   = 1.001,
    comment   = "companion to font-ini.mkiv",
    author    = "Hans Hagen, PRAGMA-ADE, Hasselt NL",
    copyright = "PRAGMA ADE / ConTeXt Development Team",
    license   = "see context related readme files",
}

-- After some experimenting with an alternative loader (one that is needed for
-- getting outlines in mp) I decided not to be compatible with the old (built-in)
-- one. The approach used in font-otn is as follows: we load the font in a compact
-- format but still very compatible with the ff data structures. From there we
-- create hashes to access the data efficiently. The implementation of feature
-- processing is mostly based on looking at the data as organized in the glyphs and
-- lookups as well as the specification. Keeping the lookup data in the glyphs is
-- very instructive and handy for tracing. On the other hand hashing is what brings
-- speed. So, the in the new approach (the old one will stay around too) we no
-- longer keep data in the glyphs which saves us a (what in retrospect looks a bit
-- like) a reconstruction step. It also means that the data format of the cached
-- files changes. What method is used depends on that format. There is no fundamental
-- change in processing, and not even in data organation. Most has to do with
-- loading and storage.

-- todo: less tounicodes

local lower = string.lower
local type, next, tonumber, tostring, unpack = type, next, tonumber, tostring, unpack
local abs = math.abs
local derivetable = table.derive
local formatters = string.formatters

local setmetatableindex   = table.setmetatableindex
local allocate            = utilities.storage.allocate
local registertracker     = trackers.register
local registerdirective   = directives.register
local starttiming         = statistics.starttiming
local stoptiming          = statistics.stoptiming
local elapsedtime         = statistics.elapsedtime
local findbinfile         = resolvers.findbinfile

----- trace_private       = false  registertracker("otf.private",        function(v) trace_private   = v end)
----- trace_subfonts      = false  registertracker("otf.subfonts",       function(v) trace_subfonts  = v end)
local trace_loading       = false  registertracker("otf.loading",        function(v) trace_loading   = v end)
local trace_features      = false  registertracker("otf.features",       function(v) trace_features  = v end)
----- trace_dynamics      = false  registertracker("otf.dynamics",       function(v) trace_dynamics  = v end)
----- trace_sequences     = false  registertracker("otf.sequences",      function(v) trace_sequences = v end)
----- trace_markwidth     = false  registertracker("otf.markwidth",      function(v) trace_markwidth = v end)
local trace_defining      = false  registertracker("fonts.defining",     function(v) trace_defining  = v end)

local report_otf          = logs.reporter("fonts","otf loading")

local fonts               = fonts
local otf                 = fonts.handlers.otf

otf.version               = 3.103 -- beware: also sync font-mis.lua and in mtx-fonts
otf.cache                 = containers.define("fonts", "otl",  otf.version, true)
otf.svgcache              = containers.define("fonts", "svg",  otf.version, true)
otf.sbixcache             = containers.define("fonts", "sbix", otf.version, true)
otf.pdfcache              = containers.define("fonts", "pdf",  otf.version, true)

otf.svgenabled            = false
otf.sbixenabled           = false

local otfreaders          = otf.readers

local hashes              = fonts.hashes
local definers            = fonts.definers
local readers             = fonts.readers
local constructors        = fonts.constructors

local otffeatures         = constructors.features.otf
local registerotffeature  = otffeatures.register

local otfenhancers        = constructors.enhancers.otf
local registerotfenhancer = otfenhancers.register

local forceload           = false
local cleanup             = 0     -- mk: 0=885M 1=765M 2=735M (regular run 730M)
local syncspace           = true
local forcenotdef         = false

local privateoffset       = fonts.constructors and fonts.constructors.privateoffset or 0xF0000 -- 0x10FFFF

local applyruntimefixes   = fonts.treatments and fonts.treatments.applyfixes

local wildcard            = "*"
local default             = "dflt"

local formats             = fonts.formats

formats.otf               = "opentype"
formats.ttf               = "truetype"
formats.ttc               = "truetype"

registerdirective("fonts.otf.loader.cleanup",       function(v) cleanup       = tonumber(v) or (v and 1) or 0 end)
registerdirective("fonts.otf.loader.force",         function(v) forceload     = v end)
registerdirective("fonts.otf.loader.syncspace",     function(v) syncspace     = v end)
registerdirective("fonts.otf.loader.forcenotdef",   function(v) forcenotdef   = v end)

-- otfenhancers.patch("before","migrate metadata","cambria",function() end)

registerotfenhancer("check extra features", function() end) -- placeholder

-- Kai has memory problems on osx so here is an experiment (I only tested on windows as
-- my test mac is old and gets no updates and is therefore rather useless.):

local checkmemory = utilities.lua and utilities.lua.checkmemory
local threshold   = 100 -- MB
local tracememory = false

registertracker("fonts.otf.loader.memory",function(v) tracememory = v end)

if not checkmemory then -- we need a generic plug (this code might move):

    local collectgarbage = collectgarbage

    checkmemory = function(previous,threshold) -- threshold in MB
        local current = collectgarbage("count")
        if previous then
            local checked = (threshold or 64)*1024
            if current - previous > checked then
                collectgarbage("collect")
                current = collectgarbage("count")
            end
        end
        return current
    end

end

function otf.load(filename,sub,instance)
    local base = file.basename(file.removesuffix(filename))
    local name = file.removesuffix(base) -- already no suffix
    local attr = lfs.attributes(filename)
    local size = attr and attr.size or 0
    local time = attr and attr.modification or 0
    -- sub can be number of string
    if sub == "" then
        sub = false
    end
    local hash = name
    if sub then
        hash = hash .. "-" .. sub
    end
    if instance then
        hash = hash .. "-" .. instance
    end
    hash = containers.cleanname(hash)
    local data = containers.read(otf.cache,hash)
    local reload = not data or data.size ~= size or data.time ~= time or data.tableversion ~= otfreaders.tableversion
    if forceload then
        report_otf("forced reload of %a due to hard coded flag",filename)
        reload = true
    end
     if reload then
        report_otf("loading %a, hash %a",filename,hash)
        --
        starttiming(otfreaders)
        data = otfreaders.loadfont(filename,sub or 1,instance) -- we can pass the number instead (if it comes from a name search)
        if data then
            -- todo: make this a plugin
            local used       = checkmemory()
            local resources  = data.resources
            local svgshapes  = resources.svgshapes
            local sbixshapes = resources.sbixshapes
            if cleanup == 0 then
                checkmemory(used,threshold,tracememory)
            end
            if svgshapes then
                resources.svgshapes = nil
                if otf.svgenabled then
                    local timestamp = os.date()
                    -- work in progress ... a bit boring to do
                    containers.write(otf.svgcache,hash, {
                        svgshapes = svgshapes,
                        timestamp = timestamp,
                    })
                    data.properties.svg = {
                        hash      = hash,
                        timestamp = timestamp,
                    }
                end
                if cleanup > 1 then
                    collectgarbage("collect")
                else
                    checkmemory(used,threshold,tracememory)
                end
            end
            if sbixshapes then
                resources.sbixshapes = nil
                if otf.sbixenabled then
                    local timestamp = os.date()
                    -- work in progress ... a bit boring to do
                    containers.write(otf.sbixcache,hash, {
                        sbixshapes = sbixshapes,
                        timestamp  = timestamp,
                    })
                    data.properties.sbix = {
                        hash      = hash,
                        timestamp = timestamp,
                    }
                end
                if cleanup > 1 then
                    collectgarbage("collect")
                else
                    checkmemory(used,threshold,tracememory)
                end
            end
            --
            otfreaders.compact(data)
            if cleanup == 0 then
                checkmemory(used,threshold,tracememory)
            end
            otfreaders.rehash(data,"unicodes")
            otfreaders.addunicodetable(data)
            otfreaders.extend(data)
            if cleanup == 0 then
                checkmemory(used,threshold,tracememory)
            end
            otfreaders.pack(data)
            report_otf("loading done")
            report_otf("saving %a in cache",filename)
            data = containers.write(otf.cache, hash, data)
            if cleanup > 1 then
                collectgarbage("collect")
            else
                checkmemory(used,threshold,tracememory)
            end
            stoptiming(otfreaders)
            if elapsedtime then
                report_otf("loading, optimizing, packing and caching time %s", elapsedtime(otfreaders))
            end
            if cleanup > 3 then
                collectgarbage("collect")
            else
                checkmemory(used,threshold,tracememory)
            end
            data = containers.read(otf.cache,hash) -- this frees the old table and load the sparse one
            if cleanup > 2 then
                collectgarbage("collect")
            else
                checkmemory(used,threshold,tracememory)
            end
        else
            data = nil
            report_otf("loading failed due to read error")
        end
    end
    if data then
        if trace_defining then
            report_otf("loading from cache using hash %a",hash)
        end
        --
        otfreaders.unpack(data)
        otfreaders.expand(data) -- inline tables
        otfreaders.addunicodetable(data) -- only when not done yet
        --
        otfenhancers.apply(data,filename,data) -- in context one can also use treatments
        --
     -- constructors.addcoreunicodes(data.resources.unicodes) -- still needed ?
        --
        if applyruntimefixes then
            applyruntimefixes(filename,data) -- e.g. see treatments.lfg
        end
        --
        data.metadata.math = data.resources.mathconstants
        --
        -- delayed tables (experiment)
        --
        local classes = data.resources.classes
        if not classes then
            local descriptions = data.descriptions
            classes = setmetatableindex(function(t,k)
                local d = descriptions[k]
                local v = (d and d.class or "base") or false
                t[k] = v
                return v
            end)
            data.resources.classes = classes
        end
        --
    end

    return data
end

-- modes: node, base, none

function otf.setfeatures(tfmdata,features)
    local okay = constructors.initializefeatures("otf",tfmdata,features,trace_features,report_otf)
    if okay then
        return constructors.collectprocessors("otf",tfmdata,features,trace_features,report_otf)
    else
        return { } -- will become false
    end
end

-- the first version made a top/mid/not extensible table, now we just
-- pass on the variants data and deal with it in the tfm scaler (there
-- is no longer an extensible table anyway)
--
-- we cannot share descriptions as virtual fonts might extend them (ok,
-- we could use a cache with a hash
--
-- we already assign an empty table to characters as we can add for
-- instance protruding info and loop over characters; one is not supposed
-- to change descriptions and if one does so one should make a copy!

local function copytotfm(data,cache_id)
    if data then
        local metadata       = data.metadata
        local properties     = derivetable(data.properties)
        local descriptions   = derivetable(data.descriptions)
        local goodies        = derivetable(data.goodies)
        local characters     = { }
        local parameters     = { }
        local mathparameters = { }
        --
        local resources      = data.resources
        local unicodes       = resources.unicodes
        local spaceunits     = 500
        local spacer         = "space"
        local designsize     = metadata.designsize or 100
        local minsize        = metadata.minsize or designsize
        local maxsize        = metadata.maxsize or designsize
        local mathspecs      = metadata.math
        --
        if designsize == 0 then
            designsize = 100
            minsize    = 100
            maxsize    = 100
        end
        if mathspecs then
            for name, value in next, mathspecs do
                mathparameters[name] = value
            end
        end
        for unicode in next, data.descriptions do -- use parent table
            characters[unicode] = { }
        end
        if mathspecs then
            for unicode, character in next, characters do
                local d = descriptions[unicode] -- we could use parent table here
                local m = d.math
                if m then
                    -- watch out: luatex uses horiz_variants for the parts
                    --
                    local italic   = m.italic
                    local vitalic  = m.vitalic
                    --
                    local variants = m.hvariants
                    local parts    = m.hparts
                    if variants then
                        local c = character
                        for i=1,#variants do
                         -- local un = variants[i].glyph
                            local un = variants[i]
                            c.next = un
                            c = characters[un]
                        end -- c is now last in chain
                        c.horiz_variants = parts
                    elseif parts then
                        character.horiz_variants = parts
                        italic = m.hitalic
                    end
                    --
                    local variants = m.vvariants
                    local parts    = m.vparts
                    if variants then
                        local c = character
                        for i=1,#variants do
                         -- local un = variants[i].glyph
                            local un = variants[i]
                            c.next = un
                            c = characters[un]
                        end -- c is now last in chain
                        c.vert_variants = parts
                    elseif parts then
                        character.vert_variants = parts
                    end
                    --
                    if italic and italic ~= 0 then
                        character.italic = italic
                    end
                    --
                    if vitalic and vitalic ~= 0 then
                        character.vert_italic = vitalic
                    end
                    --
                    local accent = m.accent -- taccent?
                    if accent then
                        character.accent = accent
                    end
                    --
                    local kerns = m.kerns
                    if kerns then
                        character.mathkerns = kerns
                    end
                end
            end
        end
        -- we need a runtime lookup because of running from cdrom or zip, brrr (shouldn't
        -- we use the basename then?)
        local filename = constructors.checkedfilename(resources)
        local fontname = metadata.fontname
        local fullname = metadata.fullname or fontname
        local psname   = fontname or fullname
        local units    = metadata.units or 1000
        --
        if units == 0 then -- catch bugs in fonts
            units = 1000 -- maybe 2000 when ttf
            metadata.units = 1000
            report_otf("changing %a units to %a",0,units)
        end
        --
        local monospaced  = metadata.monospaced
        local charwidth   = metadata.averagewidth -- or unset
        local charxheight = metadata.xheight -- or unset
        local italicangle = metadata.italicangle
        local hasitalics  = metadata.hasitalics
        properties.monospaced  = monospaced
        properties.hasitalics  = hasitalics
        parameters.italicangle = italicangle
        parameters.charwidth   = charwidth
        parameters.charxheight = charxheight
        --
        local space  = 0x0020
        local emdash = 0x2014
        if monospaced then
            if descriptions[space] then
                spaceunits, spacer = descriptions[space].width, "space"
            end
            if not spaceunits and descriptions[emdash] then
                spaceunits, spacer = descriptions[emdash].width, "emdash"
            end
            if not spaceunits and charwidth then
                spaceunits, spacer = charwidth, "charwidth"
            end
        else
            if descriptions[space] then
                spaceunits, spacer = descriptions[space].width, "space"
            end
            if not spaceunits and descriptions[emdash] then
                spaceunits, spacer = descriptions[emdash].width/2, "emdash/2"
            end
            if not spaceunits and charwidth then
                spaceunits, spacer = charwidth, "charwidth"
            end
        end
        spaceunits = tonumber(spaceunits) or units/2
        --
        parameters.slant         = 0
        parameters.space         = spaceunits            -- 3.333 (cmr10)
        parameters.space_stretch = 1*units/2   --  500   -- 1.666 (cmr10)
        parameters.space_shrink  = 1*units/3   --  333   -- 1.111 (cmr10)
        parameters.x_height      = 2*units/5   --  400
        parameters.quad          = units       -- 1000
        if spaceunits < 2*units/5 then
            -- todo: warning
        end
        if italicangle and italicangle ~= 0 then
            parameters.italicangle  = italicangle
            parameters.italicfactor = math.cos(math.rad(90+italicangle))
            parameters.slant        = - math.tan(italicangle*math.pi/180)
        end
        if monospaced then
            parameters.space_stretch = 0
            parameters.space_shrink  = 0
        elseif syncspace then --
            parameters.space_stretch = spaceunits/2
            parameters.space_shrink  = spaceunits/3
        end
        parameters.extra_space = parameters.space_shrink -- 1.111 (cmr10)
        if charxheight then
            parameters.x_height = charxheight
        else
            local x = 0x0078
            if x then
                local x = descriptions[x]
                if x then
                    parameters.x_height = x.height
                end
            end
        end
        --
        parameters.designsize    = (designsize/10)*65536
        parameters.minsize       = (minsize   /10)*65536
        parameters.maxsize       = (maxsize   /10)*65536
        parameters.ascender      = abs(metadata.ascender  or 0)
        parameters.descender     = abs(metadata.descender or 0)
        parameters.units         = units
        --
        properties.space         = spacer
        properties.encodingbytes = 2
        properties.format        = data.format or formats.otf
        properties.noglyphnames  = true
        properties.filename      = filename
        properties.fontname      = fontname
        properties.fullname      = fullname
        properties.psname        = psname
        properties.name          = filename or fullname
        --
     -- properties.name          = specification.name
     -- properties.sub           = specification.sub
        --
        properties.private       = properties.private or data.private or privateoffset
        --
        return {
            characters     = characters,
            descriptions   = descriptions,
            parameters     = parameters,
            mathparameters = mathparameters,
            resources      = resources,
            properties     = properties,
            goodies        = goodies,
        }
    end
end

-- These woff files are a kind of joke in a tex environment because one can simply convert
-- them to ttf/otf and use them as such (after all, we cache them too). The successor format
-- woff2 is more complex so there we can as well call an external converter which in the end
-- makes this code kind of obsolete before it's even used. Although ... it might become a
-- more general conversion plug in.

local converters = {
    woff = {
        cachename = "webfonts",
        action    = otf.readers.woff2otf,
    }
}

local function checkconversion(specification)
    local filename  = specification.filename
    local converter = converters[lower(file.suffix(filename))]
    if converter then
        local base = file.basename(filename)
        local name = file.removesuffix(base)
        local attr = lfs.attributes(filename)
        local size = attr and attr.size or 0
        local time = attr and attr.modification or 0
        if size > 0 then
            local cleanname = containers.cleanname(name)
            local cachename = caches.setfirstwritablefile(cleanname,converter.cachename)
            if not io.exists(cachename) or (time ~= lfs.attributes(cachename).modification) then
                report_otf("caching font %a in %a",filename,cachename)
                converter.action(filename,cachename) -- todo infoonly
                lfs.touch(cachename,time,time)
            end
            specification.filename = cachename
        end
    end
end

local function otftotfm(specification)
    local cache_id = specification.hash
    local tfmdata  = containers.read(constructors.cache,cache_id)
    if not tfmdata then

        checkconversion(specification) -- for the moment here

        local name     = specification.name
        local sub      = specification.sub
        local subindex = specification.subindex
        local filename = specification.filename
        local features = specification.features.normal
        local instance = specification.instance or (features and features.axis)
        local rawdata  = otf.load(filename,sub,instance)
        if rawdata and next(rawdata) then
            local descriptions = rawdata.descriptions
            rawdata.lookuphash = { } -- to be done
            tfmdata = copytotfm(rawdata,cache_id)
            if tfmdata and next(tfmdata) then
                -- at this moment no characters are assigned yet, only empty slots
                local features     = constructors.checkedfeatures("otf",features)
                local shared       = tfmdata.shared
                if not shared then
                    shared         = { }
                    tfmdata.shared = shared
                end
                shared.rawdata     = rawdata
             -- shared.features    = features -- default
                shared.dynamics    = { }
             -- shared.processes   = { }
                tfmdata.changed    = { }
                shared.features    = features
                shared.processes   = otf.setfeatures(tfmdata,features)
            end
        end
        containers.write(constructors.cache,cache_id,tfmdata)
    end
    return tfmdata
end

local function read_from_otf(specification)
    local tfmdata = otftotfm(specification)
    if tfmdata then
        -- this late ? .. needs checking
        tfmdata.properties.name = specification.name
        tfmdata.properties.sub  = specification.sub
        --
        tfmdata = constructors.scale(tfmdata,specification)
        local allfeatures = tfmdata.shared.features or specification.features.normal
        constructors.applymanipulators("otf",tfmdata,allfeatures,trace_features,report_otf)
        constructors.setname(tfmdata,specification) -- only otf?
        fonts.loggers.register(tfmdata,file.suffix(specification.filename),specification)
    end
    return tfmdata
end

local function checkmathsize(tfmdata,mathsize)
    local mathdata = tfmdata.shared.rawdata.metadata.math
    local mathsize = tonumber(mathsize)
    if mathdata then -- we cannot use mathparameters as luatex will complain
        local parameters = tfmdata.parameters
        parameters.scriptpercentage       = mathdata.ScriptPercentScaleDown
        parameters.scriptscriptpercentage = mathdata.ScriptScriptPercentScaleDown
        parameters.mathsize               = mathsize -- only when a number !
    end
end

registerotffeature {
    name         = "mathsize",
    description  = "apply mathsize specified in the font",
    initializers = {
        base = checkmathsize,
        node = checkmathsize,
    }
}

-- readers

function otf.collectlookups(rawdata,kind,script,language)
    if not kind then
        return
    end
    if not script then
        script = default
    end
    if not language then
        language = default
    end
    local lookupcache = rawdata.lookupcache
    if not lookupcache then
        lookupcache = { }
        rawdata.lookupcache = lookupcache
    end
    local kindlookup = lookupcache[kind]
    if not kindlookup then
        kindlookup = { }
        lookupcache[kind] = kindlookup
    end
    local scriptlookup = kindlookup[script]
    if not scriptlookup then
        scriptlookup = { }
        kindlookup[script] = scriptlookup
    end
    local languagelookup = scriptlookup[language]
    if not languagelookup then
        local sequences   = rawdata.resources.sequences
        local featuremap  = { }
        local featurelist = { }
        if sequences then
            for s=1,#sequences do
                local sequence = sequences[s]
                local features = sequence.features
                if features then
                    features = features[kind]
                    if features then
                     -- features = features[script] or features[default] or features[wildcard]
                        features = features[script] or features[wildcard]
                        if features then
                         -- features = features[language] or features[default] or features[wildcard]
                            features = features[language] or features[wildcard]
                            if features then
                                if not featuremap[sequence] then
                                    featuremap[sequence] = true
                                    featurelist[#featurelist+1] = sequence
                                end
                            end
                        end
                    end
                end
            end
            if #featurelist == 0 then
                featuremap, featurelist = false, false
            end
        else
            featuremap, featurelist = false, false
        end
        languagelookup = { featuremap, featurelist }
        scriptlookup[language] = languagelookup
    end
    return unpack(languagelookup)
end

-- moved from font-oth.lua, todo: also afm

local function getgsub(tfmdata,k,kind,value)
    local shared  = tfmdata.shared
    local rawdata = shared and shared.rawdata
    if rawdata then
        local sequences = rawdata.resources.sequences
        if sequences then
            local properties = tfmdata.properties
            local validlookups, lookuplist = otf.collectlookups(rawdata,kind,properties.script,properties.language)
            if validlookups then
             -- local choice = tonumber(value) or 1 -- no random here (yet)
                for i=1,#lookuplist do
                    local lookup   = lookuplist[i]
                    local steps    = lookup.steps
                    local nofsteps = lookup.nofsteps
                    for i=1,nofsteps do
                        local coverage = steps[i].coverage
                        if coverage then
                            local found = coverage[k]
                            if found then
                                return found, lookup.type
                            end
                        end
                    end
                end
            end
        end
    end
end

otf.getgsub = getgsub -- returns value, gsub_kind

function otf.getsubstitution(tfmdata,k,kind,value)
    local found, kind = getgsub(tfmdata,k,kind,value)
    if not found then
        --
    elseif kind == "gsub_single" then
        return found
    elseif kind == "gsub_alternate" then
        local choice = tonumber(value) or 1 -- no random here (yet)
        return found[choice] or found[1] or k
    end
    return k
end

otf.getalternate = otf.getsubstitution

function otf.getmultiple(tfmdata,k,kind)
    local found, kind = getgsub(tfmdata,k,kind)
    if found and kind == "gsub_multiple" then
        return found
    end
    return { k }
end

function otf.getkern(tfmdata,left,right,kind)
    local kerns = getgsub(tfmdata,left,kind or "kern",true) -- for now we use getsub
    if kerns then
        local found = kerns[right]
        local kind  = type(found)
        if kind == "table" then
            found = found[1][3] -- can be more clever
        elseif kind ~= "number" then
            found = false
        end
        if found then
            return found * tfmdata.parameters.factor
        end
    end
    return 0
end

local function check_otf(forced,specification,suffix)
    local name = specification.name
    if forced then
        name = specification.forcedname -- messy
    end
    local fullname = findbinfile(name,suffix) or ""
    if fullname == "" then
        fullname = fonts.names.getfilename(name,suffix) or ""
    end
    if fullname ~= "" and not fonts.names.ignoredfile(fullname) then
        specification.filename = fullname
        return read_from_otf(specification)
    end
end

local function opentypereader(specification,suffix)
    local forced = specification.forced or ""
    if formats[forced] then
        return check_otf(true,specification,forced)
    else
        return check_otf(false,specification,suffix)
    end
end

readers.opentype = opentypereader -- kind of useless and obsolete

function readers.otf(specification) return opentypereader(specification,"otf") end
function readers.ttf(specification) return opentypereader(specification,"ttf") end
function readers.ttc(specification) return opentypereader(specification,"ttf") end

function readers.woff(specification)
    checkconversion(specification)
    opentypereader(specification,"")
end

-- this will be overloaded

function otf.scriptandlanguage(tfmdata,attr)
    local properties = tfmdata.properties
    return properties.script or "dflt", properties.language or "dflt"
end

-- a little bit of abstraction

local function justset(coverage,unicode,replacement)
    coverage[unicode] = replacement
end

otf.coverup = {
    stepkey = "steps",
    actions = {
        chainsubstitution = justset,
        chainposition     = justset,
        substitution      = justset,
        alternate         = justset,
        multiple          = justset,
        kern              = justset,
        pair              = justset,
        single            = justset,
        ligature          = function(coverage,unicode,ligature)
            local first = ligature[1]
            local tree  = coverage[first]
            if not tree then
                tree = { }
                coverage[first] = tree
            end
            for i=2,#ligature do
                local l = ligature[i]
                local t = tree[l]
                if not t then
                    t = { }
                    tree[l] = t
                end
                tree = t
            end
            tree.ligature = unicode
        end,
    },
    register = function(coverage,featuretype,format)
        return {
            format   = format,
            coverage = coverage,
        }
    end
}
