convert sumti to mex operand; sample use in story arithmetic: [3 apples] + [3 apples] = what.

- namcixu
- For mo'e x
_{4}x in x_{1}there exist mo'e x_{5}y in x_{2}such that x me'au x_{3}y; for mo'e x_{6}y in x_{2}there exist mo'e x_{7}x in x_{1}such that x me'au x_{3}y.