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Optional argument presence test

I too rather like the syntax &OPTIONAL (var1 init1 predvar1) (var2 init2 predvar2).
It does present certain implementational difficulties, but I think they can be
overcome.  An interesting phenomenon is that if i<j (resp. i>j) and predvari
is null (resp. non-null) then predvarj is also.  Compiler optimizers take note!