anchor.diff:  
   * QUESTION should we show keys with hyphens instead of underscores in theming guide?
   
  +* look where else we should be using advance_page instead of start_new_page
  + ** once we fix nested keep_together blocks, we'll need to use advance_page instead of start_new_page
  +* nested keep_together blocks are broken; perhaps because we are using the same scratch document?
   * toclevels=0 should show parts only (to be consistent with HTML); currently shows nothing (#783)
   * define sum on core_ext/array to optimize for Ruby 2.4
   * allow border bottom of table header row to be set by theme
  
positive.code: * height of listing block not calculated correctly when string of contiguous characters exceeds length of line
   ** put listing block with very long line inside of sidebar; see that sidebar height is incorrect (too large)
   ** is this because source highlighting is not done in scratch document?
  * rework resolve_image_path API so it's more logical; override based on type of first argument; document as option
  * rename text-alignment attribute to text-align? (change is within alpha.14, so still a chance to change)
  * report cursor / bounding box bug in column_box
  
negative.code: * conum should never wrap (push it into the text if necessary)
  * decouple theme settings for section titles and discrete headings
  * decouple listing/literal/source theme settings; currently all under code
   ** separate theme control for listing vs literal block (and maybe source too)
  * replace explicit char ranges with classes in regexp (e.g., [[:word:]] or \w)
  * devise a way to specify a value as a string literal (variable replacement only) in theme
  * apply calculated theme values after loading?
  