##Columns and multi-arrows
##column multiple arrow space box
##Here we show block nesting. (The block diagram is essentially a nested composition of blocks.) We define two top-level blocks (in a row, left-to-right) a 'box' called 'A' and an unnamed 'col'. The latter is an invisible block where the default layout is top to bottom. Inside the 'col' we mention two new blocks (and auto define them as boxes).##In the last line we add two arrows in a single go.
box A;
space;
col {B, C;}
A -> B,C;

