bracket-not:useful — immediate primitive
( a -- a' )(
executes 0= but this word is immediate so that it does affect the cs-stack while compiling rather than compiling anything. This is useful just before words like [IF] to provide semantics of an [IFNOT]. It is most useful in conjunction with "[DEFINED] word" as it the sequence "[DEFINED] word [NOT] [IF]" can simulate "[IFNOTDEF] word"