An IDENTIFIER_NODE represents a slightly more general concept
that the standard C or C++ concept of identifier.  In particular, an
IDENTIFIER_NODE may contain a $, or other extraordinary
characters.
   
There are never two distinct IDENTIFIER_NODEs representing the
same identifier.  Therefore, you may use pointer equality to compare
IDENTIFIER_NODEs, rather than using a routine like strcmp.
   
You can use the following macros to access identifiers:
IDENTIFIER_POINTER
     char*.  This string is always NUL-terminated, and contains
no embedded NUL characters.
     IDENTIFIER_LENGTH
     IDENTIFIER_POINTER, not
including the trailing NUL.  This value of
IDENTIFIER_LENGTH (x) is always the same as strlen
(IDENTIFIER_POINTER (x)).
     IDENTIFIER_OPNAME_P
     IDENTIFIER_POINTER or the
IDENTIFIER_LENGTH.
     IDENTIFIER_TYPENAME_P
     TREE_TYPE of
the IDENTIFIER_NODE holds the type to which the conversion
operator converts.