theorem Th103:
  |-_IPC (((p => FALSUM) => FALSUM) => FALSUM) => (p => FALSUM)
proof
A1: (p => ((p => FALSUM) => FALSUM)) =>
    ((((p => FALSUM) => FALSUM) => FALSUM) => (p => FALSUM))
    in IPC-Taut by INTPRO_1:24;
  p => ((p => FALSUM) => FALSUM) in IPC-Taut by Th70,Th72;
  then (((p => FALSUM) => FALSUM) => FALSUM) => (p => FALSUM)
    in IPC-Taut by A1,INTPRO_1:def 14;
  hence thesis by Th69;
end;
