reserve i,j,n,n1,n2,m,k,l,u for Nat,
        r,r1,r2 for Real,
        x,y for Integer,
        a,b for non trivial Nat,
        F for XFinSequence,
        cF,cF1,cF2 for complex-valued XFinSequence,
        c,c1,c2 for Complex;
reserve x,y,x1,u,w for Nat;

theorem
   for y,x1,x2 being Nat holds
     y = Product ( (x2+1)+(-idseq x1)) & x2 > x1
   iff y = x1!*(x2 choose x1) & x2 > x1 by Th23;
