* IF ( value -- ) .. THEN
checks the value on the stack (at run-time, not compile-time)
and if true executes the code-piece between IF and the next
ELSE or THEN . Otherwise it has compiled a branch over
to be executed if the value on stack had been null at run-time.
( value -- )
FORTH/ANS core (dpANS.6.1.1700) compiling primitive