theorem
  for f,g being Relation, A,B being set st f|A = g|A & f|B = g|B holds
  f|(A \/ B) = g|(A \/ B)
proof
  let f,g be Relation, A,B be set;
  assume f|A = g|A & f|B = g|B;
  hence f|(A \/ B) = g|A \/ g|B by Th72
    .= g|(A \/ B) by Th72;
end;
