reserve i,j,k,n,m,l,s,t for Nat;
reserve a,b for Real;
reserve F for real-valued FinSequence;
reserve z for Complex;
reserve x,y for Complex;
reserve r,s,t for natural Number;

theorem Th21:
  s choose s = 1
proof
  reconsider m = s-s as Element of NAT by INT_1:5;
  m = 0;
  then s choose s = s choose 0 by Th20;
  hence thesis by Th19;
end;
