theorem
  ((A^Foi) \/ (B^Foi)) c= (A \/ B)^Foi
proof
  let x be object;
  assume x in ((A^Foi) \/ (B^Foi));
  then
A1: x in (A^Foi) or x in (B^Foi) by XBOOLE_0:def 3;
  (A^Foi) c= (A \/ B)^Foi & (B^Foi) c= (B \/ A)^Foi by Th25,XBOOLE_1:7;
  hence thesis by A1;
end;
