theorem Th4:
  for p being NAT-defined (the InstructionsF of S)-valued Function
  for k holds Comput(p,s,i+k) = Comput(p,Comput(p,s,i),k)
proof let p be NAT-defined (the InstructionsF of S)-valued Function;
  defpred P[Nat] means Comput(p,s,i+$1)
   = Comput(p, Comput(p,s,i),$1);
A1: now
    let k;
    assume
A2: P[k];
    Comput(p,s,i+(k+1)) = Comput(p,s,i+k+1)
      .= Following(p, Comput(p,s,i+k)) by Th3
      .= Comput(p, Comput(p,s,i),k+1) by A2,Th3;
    hence P[k+1];
  end;
A3: P[ 0];
  thus for k holds P[k] from NAT_1:sch 2(A3,A1);
end;
