reserve V, C for set;
reserve A, B, D for Element of Fin PFuncs (V, C);
reserve s for Element of PFuncs (V,C);

theorem Th20:
  mi(A ^ mi B) = mi (A ^ B)
proof
  A ^ mi B = mi B ^ A & A ^ B = B ^ A by Th3;
  hence thesis by Th19;
end;
