 reserve i,j,n,k,l for Nat;
 reserve T,S,X,Y,Z for Subset of MC-wff;
 reserve p,q,r,t,F,H,G for Element of MC-wff;
 reserve s,U,V for MC-formula;
reserve f,g for FinSequence of [:MC-wff,Proof_Step_Kinds_IPC:];
 reserve X,T for Subset of MC-wff;
 reserve F,G,H,p,q,r,t for Element of MC-wff;
 reserve s,h for MC-formula;
 reserve f for FinSequence of [:MC-wff,Proof_Step_Kinds_IPC:];
 reserve i,j for Element of NAT;
 reserve F1,F2,F3,F4,F5,F6,F7,F8,F9,F10,G for MC-formula;
 reserve x1,x2,x3,x4,x5,x6,x7,x8,x9,x10,x for Element of MC-wff;
reserve x1,x2,x3,x4,x5,x6,x7,x8,x9,x10 for object;

theorem Th104:
  |-_IPC (p => FALSUM) 'equiv' (((p => FALSUM) => FALSUM) => FALSUM)
proof
A1: |-_IPC (p => FALSUM) => (((p => FALSUM) => FALSUM) => FALSUM) by Th102;
  |-_IPC (((p => FALSUM) => FALSUM) => FALSUM) => (p => FALSUM) by Th103;
  hence thesis by A1,Th84;
end;
