theorem Th1:
  for a,b,c,d being Complex st b <> 0 & a/b = c/d holds a = b*c/d
  proof
    let a,b,c,d be Complex;
    assume that
A1: b <> 0 and
A2: a/b = c/d;
    thus a = b*(a/b) by A1,XCMPLX_1:87
    .= b*c/d by A2,XCMPLX_1:74;
  end;
