Rule used to change bound variables, using implicit substitution. Usage of this theorem is discouraged because it depends on [ax-13]. See cbvexvw for a version requiring fewer axioms, to be preferred when sufficient. (Contributed by NM, 21-Jun-1993.) Remove dependency on [ax-10] and shorten proof. (Revised by Wolf Lammen, 11-Sep-2023.) (New usage is discouraged.)