theorem Th99:
  |-_IPC ((p 'or' q) => FALSUM) 'equiv' ((p => FALSUM) '&' (q => FALSUM))
proof
A1: |-_IPC ((p 'or' q) => FALSUM) => ((p => FALSUM) '&' (q => FALSUM))
      by Th97;
  |-_IPC ((p => FALSUM) '&' (q => FALSUM)) => ((p 'or' q) => FALSUM) by Th98;
  hence thesis by A1,Th84;
end;
