class Arbre::HTML::Tag

def indent(open_tag, child_content, close_tag)
  spaces = ' ' * indent_level * INDENT_SIZE
  html = ""
  if no_child? || child_is_text?
    if self_closing_tag?
      html << spaces << open_tag.sub( />$/, '/>' )
    else
      # one line
      html << spaces << open_tag << child_content << close_tag
    end
  else
    # multiple lines
    html << spaces << open_tag << "\n"
    html << child_content # the child takes care of its own spaces
    html << spaces << close_tag
  end
  html << "\n"
  html
end