theorem Th16:
  p in L~f iff not p in LeftComp f & not p in RightComp f
proof
A1: p in L~f iff not p in (L~f)` by XBOOLE_0:def 5;
  (L~f)` = LeftComp f \/ RightComp f by GOBRD12:10;
  hence thesis by A1,XBOOLE_0:def 3;
end;
