theorem Th24:
  (F /\ G)"" c= (F"") /\ (G"")
proof
  let i;
  assume i in (F/\G)"";
  then consider w such that
A1: i = w" and
A2: w in F /\ G;
  w in G by A2,XBOOLE_0:def 4;
  then
A3: w" in G"";
  w in F by A2,XBOOLE_0:def 4;
  then w" in F"";
  hence thesis by A1,A3,XBOOLE_0:def 4;
end;
