Words for conditional interpretation and compilation

These words do not allow nesting, in a deviation from Forth 2012.

Skip until else or end are seen in the input stream

: [else]       begin
                  begin
                     bl word count dup while
                     2dup s" [then]" stri= if 2drop exit then
                     s" [else]" stri= if exit then
                  repeat 2drop
               refill 0= until
               ; immediate

If the top of the stack is false, skip processing input data until else or then are seen

: [if]         0= if postpone [else] then ; immediate

No-operation, at the end of an if/else block.

: [then]       ; immediate

If the word following undefined is not found, leave false on the stack.

: [undefined]  bl word find nip 0= ; immediate