reserve T, X, Y for Subset of MC-wff;
reserve p, q, r, s for Element of MC-wff;
reserve T, X, Y for Subset of MC-wff;
reserve p, q, r for Element of MC-wff;
reserve T, X, Y for Subset of MC-wff;
reserve p, q, r for Element of MC-wff;

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;
