theorem
  for n st n = ((a,b) In_Power (k+l)).(k+1) & l>0 holds a divides n
  proof
    let n;
    assume n = ((a,b) In_Power (k+l)).(k+1) & l>0; then
    consider x such that
    A2: n = a*x by Th50;
    thus thesis by A2;
  end;
