theorem Th126:
 |-_IPC p => ((p => FALSUM) => q)
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
A3: {p => FALSUM,p} |-_IPC FALSUM by A1,Th27;
  {p => FALSUM,p} |-_IPC FALSUM => q by Th26; then
  {p => FALSUM,p} |-_IPC q by A3,Th27; then
  {p} |-_IPC (p => FALSUM) => q by Th55;
  hence thesis by Th54;
end;
