theorem Th35:
  n = {} or ex m st n = succ m
proof
  defpred P[Nat] means $1 = {} or ex m st $1 = succ m;
A1: for a being Nat st P[a] holds P[succ a];
A2: P[0];
  thus P[n] from ORDINAL2:sch 17(A2,A1);
end;
