def pruntLayer(l,w):
    Keep=sorted(Open[l])[:w] # best `w` node from `Open[l]`
    Prune = [n for n in Open[l] if n not in Keep]
    beam_stack.top().f_max=min([f(n) for n in Prune])
    for n in Prune:
        Open[l].remove(n)