one of the rare words whose stack-change is
condition-dependet. This word will duplicate
the value only if it is not zero. The usual
place to use it is directly before a control-word
that can go to different places where we can
spare an extra DROP on the is-null-part.
This makes the code faster and often a little
easier to read.
example:
: XX BEGIN ?DUP WHILE DUP . 2/ REPEAT ; instead of
: XX BEGIN DUP WHILE DUP . 2/ REPEAT DROP ;