reserve a,b,c,d for Real;
reserve r,s for Real;

theorem
  a/b > 0 implies b > 0 & a > 0 or b < 0 & a < 0
proof
  assume
A1: a/b>0;
  then
A2: a <> 0;
  a/b = a*b" by XCMPLX_0:def 9;
  then b" <> 0 by A1;
  hence thesis by A1,A2;
end;
