reserve n for Nat,
        i,j,i1,i2,i3,i4,i5,i6 for Element of n,
        p,q,r for n-element XFinSequence of NAT;
reserve i,j,n,n1,n2,m,k,l,u,e,p,t for Nat,
        a,b for non trivial Nat,
        x,y for Integer,
        r,q for Real;

theorem Th20:
  Px(a,k) = Px(a,n) implies k = n
proof
  k < n or n <k or k=n by XXREAL_0:1;
  hence thesis by Th19;
end;
