theorem Th72:
  |-_IPC p => ((p => FALSUM) => FALSUM)
proof
   p => FALSUM in {p => FALSUM,p} by TARSKI:def 2; then
A1: {p => FALSUM,p} |-_IPC p => FALSUM by Th67;
   p in {p => FALSUM,p} by TARSKI:def 2; then
  {p => FALSUM,p} |-_IPC p by Th67; then
  {p => FALSUM,p} |-_IPC FALSUM by A1,Th27; then
  {p} |-_IPC (p => FALSUM) => FALSUM by Th55; then
  |-_IPC p => ((p => FALSUM) => FALSUM) by Th54;
  hence thesis;
end;
