reserve E,X,Y,x for set;
reserve A,B,C for Subset of E;

theorem
  (A \ B)` = A` \/ B
proof
  B in bool E by Def1;
  then
A1: B c= E by ZFMISC_1:def 1;
  thus (A \ B)` = (E \ A) \/ E /\ B by XBOOLE_1:52
    .= A` \/ B by A1,XBOOLE_1:28;
end;
