Description: Hypothesis builder for elementhood. (Contributed by NM, 1-Aug-1993) (Revised by Mario Carneiro, 11-Aug-2016) (Proof shortened by Wolf Lammen, 16-Nov-2019)

|- F/_ x A

|- F/_ x B

|- F/ x A e. B

|- ( T. -> F/_ x A )

|- ( T. -> F/_ x B )

|- ( T. -> F/ x A e. B )