theorem Th11:
  0 < m & m*i divides m implies i = 1 or i = -1
  proof
    assume that
A1: 0 < m and
A2: m*i divides m;
    m divides m*i;
    then m = m*i or m = ---m*i by A2,INT_2:11;
    hence i = 1 or i = -1 by A1,XCMPLX_1:7,181;
  end;
