
theorem
  for L being non empty RelStr, X1, X2, Y1, Y2 being Subset of L st X1
  c= Y1 & X2 c= Y2 holds X1 "/\" X2 c= Y1 "/\" Y2
proof
  let L be non empty RelStr, X1, X2, Y1, Y2 be Subset of L such that
A1: X1 c= Y1 & X2 c= Y2;
  let q be object;
  assume q in X1 "/\" X2;
  then ex x, y being Element of L st q = x "/\" y & x in X1 & y in X2;
  hence thesis by A1;
end;
