=/ pins a named value (a face) into the subject for the lines below it.

name=value optionally declares the type.

=/(a 3 (add a a)) is 6.

unit is the maybe type: either ~ or [~ value].

(unit @) holds an atom or nothing.

?~ branches on whether a value is ~ (null): first branch if it is, second if not.

=/(a=(unit @) ~ ?~(a 0 u.a)) is 0.