 reserve x for Real,
    p,k,l,m,n,s,h,i,j,k1,t,t1 for Nat,
    X for Subset of REAL;
reserve x for object, X,Y,Z for set;
 reserve M,N for Cardinal;
reserve X for non empty set,
  s for sequence of X;

theorem
  for n being natural Number holds n is non zero implies n = 1 or n > 1
proof
  let n be natural Number;
  assume n is non zero;
  then 0+1 <= n by Th13;
  hence n = 1 or n > 1 by XXREAL_0:1;
end;
