while-statement=

  -+-----------------------+--> WHILE ---> predicate -----+
   +-> <beginning-label>: -+                              |
  +-------------------------------------------------------+
  +-+-> DO -+-> compound-use-statement -+--> END WHILE -+-+
    |       +-------------<-------------+               | |
    +-> LOOP -+> compound-use-statement -+-> END LOOP --+ |
              +-------------<------------+                |
  +-------------------------<-----------------------------+
  +-+--------------------+------------------------------------>
    +-> <ending-label> --+