prefix-begin:useful — immediate primitive
takes the execution token of the following word and saves it on the compile-stack. The correspondig closing ) will then feed it into >COMPILE - so this pair of word provides you with a prefix-operation syntax that you may have been seen in lisp-like languages.
($ IF ($ 0= A1 @ )) ($ THEN ." hello " )
Note that an opening simple ( paren is a comment.