undefinedp - determine whether or not a given value is undefined.

int undefinedp( mixed arg );

Return 1 if `arg' is undefined.  'arg' will be undefined in the following
cases:

<DL>
* it is a variable set equal to the return value of a call_other to a
non-existent method (e.g. arg = call_other(obj, "???")).
* it is a variable set equal to the return value of an access of an
element in a mapping that doesn't exist (e.g. arg = map[not_there]).
* it has not yet been initialized.
* it points to a destructed object.
* it is a function (formal) parameter that corresponds to a missing actual argument.
</DL>

 Tim Hollebeek  Beek@ZorkMUD, Lima Bean, IdeaExchange, and elsewhere
