If you're busy, I can clean this up and apply it.
I wonder if it would be better to have lastval() return the last value
returned by nextval() or setval() for the current session, regardless of
any intervening DROP SEQUENCE commands. This would simplify the
implementation (we can just store the int8 value produced by the last
nextval() / setval() rather than a pointer to the sequence object
itself), although it is debatable whether this behavior is more logical
or not. Comments?
-Neil