 reserve x for Real,
    p,k,l,m,n,s,h,i,j,k1,t,t1 for Nat,
    X for Subset of REAL;

theorem Th6:
  for i being natural Number holds i = 0 or ex k st i = k + 1
proof
  let i be natural Number;
A0: i is Nat by TARSKI:1;
  defpred P[natural Number] means $1 = 0 or ex k st $1 = k + 1;
A1: P[h] implies P[h + 1];
A2: P[0];
  for i holds P[i] from NatInd(A2,A1);
  hence thesis by A0;
end;
