theorem Th89:
  |-_IPC ((p '&' q) => FALSUM) 'equiv' (p => (q => FALSUM))
proof
A1: |-_IPC ((p '&' q) => FALSUM) => (p => (q => FALSUM)) by Th87;
  |-_IPC (p => (q => FALSUM)) => ((p '&' q) => FALSUM) by Th88;
  hence thesis by A1,Th84;
end;
