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Quick query about CONSTANTP

Sometimes it is useful to know whether a symbol is a constant
(for example, T, NIL, PI, MOST-POSITIVE-FIXNUM, :DIRECTORY, and so on).
How about a function CONSTANTP for this purpose?  (In particular,
the evaluator needs it for error-checking.)  It could be implemented
as a property-list chek, though for speed it would be nice if an
implementation could dig up a bit in the symbol itself.  This bit
would be set by DEFCONSTANT and when interning in the keyword package.
--Guy