# rail_slots.ludic - the rail's places: made for a count, emptied in place
# the rail emptied in place: made again only when it is to hold another count
function hn_empty(hints_st: mut HintsState) -> void {
  if len(hints_st.hn_top) != hints_st.hn_shown {
    hints_st.hn_top = hn_slots(hints_st.hn_shown)
    return
  }
  for i in 0 .. len(hints_st.hn_top) { hints_st.hn_top[i] = -1 }
}
# a rail of n empty places
function hn_slots(n: int) -> []int {
  let out = new []int
  for i in 0 .. n { push(out, -1) }
  return out
}