reserve a, b, n for Nat,
  r for Real,
  f for FinSequence of REAL;
reserve p for Prime;

theorem Th1:
  for a,b,c,d being Integer st a divides c & b divides d holds a*b divides c*d
  proof
  let a, b, c, d be Integer;
  given x being Integer such that
A1: c = a*x;
  given y being Integer such that
A2: d = b*y;
  take x*y;
  thus thesis by A1,A2;
end;
