(add (mul 2 3) 4)
calls nest: an argument can itself be a call.
the innermost calls run first.
glossary
%: calls a gate with several arguments, gathering them into one sample.
%:(add 2 3) is 5.
irregular forms:
(gate a b c) is %:(gate a b c)
add sums two atoms.
(add 2 3) is 5.
mul multiplies two atoms.
(mul 3 4) is 12.