# File lib/facets/more/pqueue.rb, line 258
  def replace_top(v)
    # replace top element
    if empty?
      @qarray[1] = v
      @size += 1
      return nil
    else
      res = @qarray[1]
      @qarray[1] = v
      downheap(1)
      return res
    end
  end