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Re: Snapped Elastic

Actually, I didn't even have in mind all the hair of actual versus
allocated sizes, only a primitive that could shrink a vector, but never
grow it.
What would you think of a medium-level primitive that would attempt
to resize a vector, returning it is it succeeded and returning false
if it failed?  Then different implementations could have different
criteria for whether it could resize a vector in place.  User-level
code would do:
      (DEFUN VECTOR-RESIZE (VEC NEWSIZE)	;may not return EQ pointer
                 (DO ((V (MAKE-VECTOR NEWSIZE))
                      (N (MIN (VECTOR-LENGTH VEC) NEWSIZE))
                      (J 0 (+ J 1)))
                     ((= J N) V)
                   (VSET (VREF VEC J) V J))))
Then whatever cases are handled well by the low-level guts would be speedy
and result in an EQ pointer, and otherwise the user code would have to
deal with the copying.  Some implementations would never allow resizing;
some would always win, by using forwarding pointers; some would win when
decreasing but not increasing; some would win for any variation below
the initially allocated size; and so on.