theorem Th21:
  Cn(X) \/ Cn(Y) c= Cn(X \/ Y)
proof
A1: Cn(Y) c= Cn(X \/ Y) by CQC_THE1:18,XBOOLE_1:7;
  Cn(X) c= Cn(X \/ Y) by CQC_THE1:18,XBOOLE_1:7;
  hence thesis by A1,XBOOLE_1:8;
end;
