Description: Obsolete proof of exan as of 19-Jun-2023. (Contributed by NM, 18-Aug-1993)(Proof shortened by Andrew Salmon, 25-May-2011)(Proof
shortened by Wolf Lammen, 13-Jan-2018) Reduce axiom dependencies.
(Revised by BJ, 7-Jul-2021)(Proof shortened by Wolf Lammen, 6-Nov-2022)(Proof modification is discouraged.)(New usage is discouraged.)