theorem
  for I,Y being non empty set, M be Y-valued ManySortedSet of I,
     x be Element of I
  holds M.x = M/.x
proof
  let I,Y be non empty set, M be Y-valued ManySortedSet of I,
     x be Element of I;
  dom M = I by PARTFUN1:def 2;
  hence M.x = M/.x by PARTFUN1:def 6;
end;
