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

theorem
  a*b > 0 implies a > 0 & b > 0 or a < 0 & b < 0
proof
  assume
A1: a*b>0;
  then
A2: b <> 0;
  a <> 0 by A1;
  hence thesis by A1,A2;
end;
