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

theorem Th8:
  (p => r) => ((q => r) => ((p 'or' q) => r)) in CnIPC (X)
proof
  T is IPC_theory & X c= T implies (p => r) => ((q => r) => ((p 'or' q) =>
  r)) in T;
  hence thesis by Def15;
end;
