* [NOT] ( 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"

useful immediate primitive