Wild idea: Instead of sentinels, have a way of declaring optional
arguments with no default, and a way of conditionally assigning a
value to them if they are not bound.

E.g.

     def order(eggs = 4, spam =):
         spam ?= Spam()

Here the '?=' means "if spam is not bound, then evaluate the
rhs and assign it, otherwise do nothing."

--
Greg
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