Description: A special instance of sp applied to an equality with a disjoint
variable condition. Unlike the more general sp , we can prove this
without ax-12 . Instance of aeveq .

The antecedent A. x x = y with distinct x and y is a
characteristic of a degenerate universe, in which just one object
exists. Actually more than one object may still exist, but if so, we
give up on equality as a discriminating term.

Separating this degenerate case from a richer universe, where inequality
is possible, is a common proof idea. The name of this theorem follows a
convention, where the condition A. x x = y is denoted by 'aev', a
shorthand for 'all equal, with a distinct variable condition'.
(Contributed by Wolf Lammen, 14-Mar-2021)