def to_h(nonil=true)
      return {} unless @marks
      marks = @marks.dup
      if nonil
        marks.delete_if {|k,v| v.nil?}
      end
      marks
    end
def to_h(nonil=true)
      return {} unless @marks
      marks = @marks.dup
      if nonil
        marks.delete_if {|k,v| v.nil?}
      end
      marks
    end
Converts this Markable to a Hash. When _nonil_ is true, nil mark values
 do not lead to hash entries.