def start_document version, tag_directives, implicit
      n = Nodes::Document.new version, tag_directives, implicit
      set_start_location(n)
      @last.children << n
      push n
    end
def start_document version, tag_directives, implicit
      n = Nodes::Document.new version, tag_directives, implicit
      set_start_location(n)
      @last.children << n
      push n
    end
Handles start_document events with +version+, +tag_directives+,
 and +implicit+ styling.

 See Psych::Handler#start_document