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
  s>=1 & t = s-1 implies s choose t = s
proof
  assume that
A1: s>=1 and
A2: t = s-1;
  s choose t = s choose 1 by A1,A2,Th20;
  hence thesis by A1,Th23;
end;
