Trying to implement FPOSITION with LAMBDA-MACROs.

[Richard M. Stallman <RMS at MIT-AI>]

LAMBDA-MACRO is a good hack but is not exactly what JONL was suggesting.

The idea of FPOSITION is that ((FPOSITION X Y) MORE ARGS)
expands into (FPOSITION-INTERNAL X Y MORE ARGS), and
((FPOSITION) MORE ARGS) into (FPOSITION-INTERNAL NIL NIL MORE ARGS).
In JONL's suggestion, the expander for FPOSITION operates on the
entire form in which the call to the FPOSITION-list appears, not
just to the FPOSITION-list.  This allows FPOSITION to be handled
straightforwardly; but also causes trouble with (FUNCTION (FPOSITION
...)) where lambda-macros automatically work properly.

It is possible to define FPOSITION using lambda-macros by making
(FPOSITION X Y) expand into
(LAMBDA (&REST ARGS) (FUNCALL* 'FPOSITION-INTERNAL X Y ARGS))
but this does make worse code when used in an internal lambda.
It would also be possible to use an analogous SUBST function
but first SUBST functions have to be made to work with &REST args.
I think I can do this, but are SUBST functions in Common Lisp?