reserve Q,Q1,Q2 for multLoop;
reserve x,y,z,w,u,v for Element of Q;

theorem Th9:
  a_op(x,y,z) = 1.Q implies x*(y*z) = (x*y)*z
proof
  assume a_op(x,y,z) = 1.Q;
  then (x*(y*z)) * 1.Q = ((x*y)*z);
  hence thesis;
end;
