Description: Rule to change the bound variable in a maps-to function, using implicit
substitution. This version has bound-variable hypotheses in place of
distinct variable conditions. (Contributed by NM, 11-Sep-2011) Add
disjoint variable condition to avoid ax-13 . See cbvmptg for a less
restrictive version requiring more axioms. (Revised by Gino Giotto, 17-Jan-2024)