: _|& ( n1 n2 cell ) >R >R DUP R> R> ( n1 n1 n2 cell ) @ EXECUTE ( n1 f ) ; ( n1 f n2 -- n1 f ) : _| CREATE , DOES> ( n1 f n2 cell ) ROT IF 2DROP 1 EXIT THEN ( n1 true ) _|& ;