WHILE
* WHILE ( cond -- )

middle part of a >BEGIN .. >WHILE .. >REPEAT
control-loop - if cond is true the code-piece up to >REPEAT
is executed which will then jump back to >BEGIN - and if
the cond is null then >WHILE will branch to right after
the >REPEAT
(compare with >UNTIL that forms a >BEGIN .. >UNTIL loop)

core compiling primitive