defined HtmlFormat#push_visited_results()
use HtmlFormat#push_visited_results()
moved some operations in LinkNodeFormatter#visit() into #get_caption()
introduced DescLeafFormatter#split_into_parts()
moved DescLeafFormatter#split_into_parts() to HtmlFormat
changed the return value of HtmlFormat#split_into_parts()
a refactoring of LinkNodeFormatter#get_caption()
replaced join("") with join