Description: A set does not contain the singleton formed on it. More precisely, one
can prove that a class contains the singleton formed on it if and only if
it is proper and contains the empty set (since it is "the singleton formed
on" any proper class, see snprc ):
|- -. ( { A } e. A <-> ( (/) e. A -> A e. _V ) ) . (Contributed by BJ, 4-Feb-2023)