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HELP ISCONSTANT                         Jonathan Laventhol, August 1984.

    POP11 programs can have variables (see *VARS) and constants (see
*CONSTANT).  Sometimes it is useful to be able to find out if some word has
been declared as a constant, and you can do so like this:

        isconstant("isconstant") =>
        ** <true>

    Constants can only be assigned to once.  Given a "fresh" constant which
has not yet been assigned to, this procedure returns the word "undef".  It
returns true for a constant which has been given a value, and false for
anything else.