An identifier is a combination of letters, digits, dollar  signs
  ($),   and   underscores  (_)  that  conform  to  the  following
  restrictions:

   o  An identifier cannot start with a digit.

   o  An identifier cannot contain any space or special symbols.

   o  The first 31 characters must denote a unique name within the
      block  in  which  the identifier is declared.  An identifier
      longer than 31 characters generates a warning  message;  the
      compiler   ignores   characters   beyond   the  thirty-first
      character.  An  identifier  cannot  start  or  end  with  an
      underscore, nor can two adjacent