anchor.diff:   ** add broader character range to monospace font so we can drop fallback font by default (#282)
    ** consider having a fallback for prose and fallback for literal
   * should we drop "<font>" handler in formatted text transform?
  +* FILE ISSUE: ttfunk 1.5.0 chokes on narrow no-break space char \u202f
   * implement text transform for running content (#702)
   * implement start line number in source listing (rouge) (#752)
    ** shouldn't this be fixed in core at the same time? (core #1742)
   * implement line highlighting in rouge (#681)
  +* remove hack to add trailing endline to source for rouge
  +* should we allow the template/format of the line number in the source listing to be controlled using a sprintf template?
   * only nest outline inside of root if doctype=article (still valid?)
   * warn if image in running content can't be found (#731)
    ** only report failure to embed image in running content once
  
positive.code: * space around inline anchors/index entries doesn't get collapsed by text formatter
  * add support for format attribute on image macro to image-related attributes such as title-page-background-image
   ** support explicit image format for cover page image and page background image
  * introduce abstract-title attribute to complement preface-title?
  * need some sort of post_construct method for converter that receives document
   ** inline convert methods can get called before init_pdf
  
negative.code: 
  * QUESTION should we show keys with hyphens instead of underscores in theming guide?
  
  * 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
  * implement SVG fit for background image (#765)
  * vertically center background image by default (yes?)
  * stroke and fill multi-page sidebar block (#259) and example block (#362)
   ** see PR
  * part/chapter/section start pages are calculated twice, once for each running content; should be shared
   ** probably need to introduce a model object to hold the cache
  * UPSTREAM: highlighting of CSS w/ rouge seems messed up; don't recognize keywords
  