theorem Th80:
  |-_IPC ((p => FALSUM) 'or' q) => (p => q)
proof
A1: (p => FALSUM) =>(p => q) in IPC-Taut by Th70,Th76;
  q => (p => q) in IPC-Taut by INTPRO_1:def 14;
  then A2: ((p => FALSUM) =>(p => q)) '&' (q => (p => q)) in IPC-Taut
    by A1,INTPRO_1:34;
  (((p => FALSUM) =>(p => q)) '&' (q => (p => q))) =>
  (((p => FALSUM)'or' q) => (p => q)) in IPC-Taut by Th70,Th77;
  then ((p => FALSUM)'or' q) => (p => q) in IPC-Taut
    by A2,INTPRO_1:def 14;
  hence thesis by Th69;
end;
