smart-word ansi .6.1.1700
<CORE>
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.
reference: p4_if in ../src/core.c:1321, export CS IF