theorem
  CnS4(CnS4(X)) = CnS4(X)
proof
A1: CnS4(X) c= CnS4(CnS4(X)) by Th89;
  CnS4(CnS4(X)) c= CnS4(X) by Lm23;
  hence thesis by A1,XBOOLE_0:def 10;
end;
