a rule is a parser. dem parses decimal digits. rash runs a rule over a whole cord, and crashes if it does not match.
parse input.
=> sets the subject to its head, then evaluates its tail against it.
=>([a=1 b=2] b) is 2.
|% makes a core: a battery of named arms, each written ++ name.
reach an arm back out with =< or arm:core.
=< a |% ++ a 5 --
is 5.
%= resolves a wing with some faces changed.
=/(a [x=1 y=2] a(x 9)) is [x=9 y=2].
irregular forms:
a(x 9) is %=(a x 9)
$(a b) restarts a loop with a changed to b