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 Th89:
  X c= CnS4(X)
proof
  let a be object;
  assume
A1: a in X;
  then reconsider t = a as Element of MC-wff;
  for T st T is S4_theory & X c= T holds t in T by A1;
  hence thesis by Def23;
end;
