Reserved_Words

 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.