reserve A for set;
reserve X,Y,Z for set,x,x1,x2,y,y1,y2,z,z1,z2 for object;
reserve u for UnOp of A,
  o,o9 for BinOp of A,
  a,b,c,e,e1,e2 for Element of A;

theorem Th7:
  o is commutative implies (e is_a_unity_wrt o iff e is_a_right_unity_wrt o)
by Th5;
