theorem Th46:
  (p => r) => ((q => r) => ((p 'or' q) => r)) is valid_IPC
proof
  (p => r) => ((q => r) => ((p 'or' q) => r)) in IPC-Taut
  proof
    IPC-Taut is IPC_theory;
    hence thesis;
  end;
  hence thesis;
end;
