reserve e for set;

theorem
  for I1,I2 being non empty set, M being ManySortedSet of [:I1,I2:], o1
  being Element of I1, o2 being Element of I2 holds (~M).(o2,o1) = M.(o1,o2)
proof
  let I1,I2 be non empty set, M be ManySortedSet of [:I1,I2:], o1 be Element
  of I1, o2 be Element of I2;
  [o1,o2] in [:I1,I2:];
  then [o1,o2] in dom M by PARTFUN1:def 2;
  hence thesis by FUNCT_4:def 2;
end;
