* 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.
IF
( value -- )
.. THEN
>IF
>ELSE
>THEN
core compiling primitive