Description: Rule used to change bound variables, using implicit substitution. Usage
of this theorem is discouraged because it depends on ax-13 . Use the
weaker cbv3hv if possible. (Contributed by NM, 8-Jun-1993)(Proof
shortened by Andrew Salmon, 25-May-2011)(Proof shortened by Wolf
Lammen, 12-May-2018)(New usage is discouraged.)