def prune
      return true if !@down.nil? && down.prune 
      return true if !@left.nil? && left.prune 
      return true if !@right.nil? && right.prune 
      return true unless @value.nil?
      up.down = nil if !up.nil? && left.nil? && right.nil?
      false
    end
def prune
      return true if !@down.nil? && down.prune 
      return true if !@left.nil? && left.prune 
      return true if !@right.nil? && right.prune 
      return true unless @value.nil?
      up.down = nil if !up.nil? && left.nil? && right.nil?
      false
    end
Prune the tree back from this node onward so that all leaves have values.