theorem Th34:
  (A \ B)"" = (A"") \ (B"")
proof
  let a;
  hereby
    assume a in (A\B)"";
    then
A1: a" in A\B by Th29;
    then not a" in B by XBOOLE_0:def 5;
    then
A2: not a in B"" by Th29;
    a in A"" by A1,Th29;
    hence a in (A"") \ (B"") by A2,XBOOLE_0:def 5;
  end;
  assume
A3: a in (A"") \ (B"");
  then not a in B"" by XBOOLE_0:def 5;
  then
A4: not a" in B by Th29;
  a" in A by A3,Th29;
  then a" in A\B by A4,XBOOLE_0:def 5;
  hence thesis by Th29;
end;
