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 Th19:
  s choose 0 = 1
proof
  reconsider m = s-0 as Element of NAT by ORDINAL1:def 12;
  m = s;
  then (s choose 0) = (s!)/(1 * (s!)) by Def3,Th12
    .= 1 by XCMPLX_1:60;
  hence thesis;
end;
