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

theorem Th6:
  p => (p 'or' q) in CnIPC (X)
proof
  T is IPC_theory & X c= T implies p => (p 'or' q) in T;
  hence thesis by Def15;
end;
