reserve A,B,C for Ordinal,
  X,X1,Y,Y1,Z for set,a,b,b1,b2,x,y,z for object,
  R for Relation,
  f,g,h for Function,
  k,m,n for Nat;
reserve M,N for Cardinal;
reserve S for Sequence;

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;
