Reserved words are words that are  reserved  for  the  names  of
  statements,  data  types,  directives,  identifiers, specifiers,
  statements,  and   operators.    You   cannot   redefine   these
  identifiers.   Examples of reserved words include AND, END, NOT,
  IF, and WHILE.

  Redefinable reserved  words  are  used  to  name  operators  and
  identifiers.  You can redeclare these words, but, if you do, the
  language feature becomes unavailable within the block  in  which
  you  redeclare  the  word.   The  redefinable reserved words are
  AND_THEN, BREAK,  CONTINUE,  MODULE,  OR_ELSE,  OTHERWISE,  REM,
  RETURN, VALUE, and VARYING.