theorem
  M |= F1 & M |= F2 implies M |= F1 \/ F2
proof
  assume
A1: M |= F1 & M |= F2;
  let H;
  assume H in F1 \/ F2;
  then H in F1 or H in F2 by XBOOLE_0:def 3;
  hence thesis by A1;
end;
