a.modify(_.b.c, _.d.e) => new PathMod(a, << chained modifications >>)
modifyAll(a)(_.b.c, _.d.e) => new PathMod(a, << chained modifications >>)
a.modify(_.b.c) => new PathMod(a, (A, F) => A.copy(b = A.b.copy(c = F(A.b.c))))
A helper method for modifyPimp_impl and modifyAllPimp_impl.
modify(a)(_.b.c) => new PathMod(a, (A, F) => A.copy(b = A.b.copy(c = F(A.b.c))))