
theorem
  for X being set, f being Function st f c= id X holds X-indexing f = id X
proof
  let X be set, f be Function;
  assume f c= id X;
  then (id X)+*f = id X by FUNCT_4:98;
  hence X-indexing f = X-indexing id X by Th12
    .= id X;
end;
