anchor.diff:   ** using fallback fonts significantly slows down Prawn because it checks every letter every time (see https://github.com/prawnpdf/prawn/blob/master/lib/prawn/text/formatted/box.rb#L427-L434)
    ** 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
  +* transform table cell text inside correct font context (allows us to check the current font, such as in breakable_uri)
   * should we drop "<font>" handler in formatted text transform?
  -* FILE ISSUE: ttfunk 1.5.0 chokes on narrow no-break space char \u202f
  +* ttfunk 1.5.0 chokes on narrow no-break space char \u202f (patch applied; https://github.com/prawnpdf/ttfunk/issues/39)
   * 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)
  
positive.code: 
  * 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
  * 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
   ** compare with pygments
   ** rouge also incorrectly identifies an endline in a schemaLocation attribute as an error
  
negative.code: * allow color and string template to be specified for alt text (#730)
  * if image fails to load, could draw a box with an X in it to indicate it's broken
  * enable fit=scale-down (or fit=contain) for title logo image; otherwise, just let it get clipped (will require passing at option to embed_image to prevent page from advancing)
  * add option to force margin (to apply even when at top of page)
  * make section depth for section-title attribute in running content configurable (#726)
  * add ability to set watermark_image which goes on top of each (non-imported) page instead of bottom (#727)
  * AsciiDoc table cell leaves padding below last block (due to no margin collapsing)
  