Returns one of a series of values depending on the evaluation of
       a series of conditional (Boolean) expressions.
       Format
          CHOICE [OF]
               boolean-expression-1 [THEN] value-1
               [boolean-expression-2 [THEN] value-2]
                       .              .     .
                       .              .     .
                       .              .     .
               ELSE value-n
          END_CHOICE