theorem Th22:
  Cn(X \/ Y) = Cn(Cn(X) \/ Cn(Y))
proof
A1: Cn(X \/ Y) c= Cn(Cn(X) \/ Cn(Y)) by Lm1,CQC_THE1:18;
  Cn(Cn(X) \/ Cn(Y)) c= Cn(X \/ Y) by Th2,Th21;
  hence thesis by A1,XBOOLE_0:def 10;
end;
