anchor.diff:    - later on, initialize_wrap drops fragment with empty text
   * use term "page number label" to refer to the visible, printed page number (not the implicit page number)
   
  +== Known Issues
  +
  +* inline image at start of the line is slighly shifted to the right due to the fact that it's placed in the center of the reserved fragment width; perhaps we are adding this padding
  +
   == Potential Optimizations
   
   * if autofit is set on a listing/literal block that has conums, we are splitting fragments by line twice
  
positive.code: * document all permutations of image sizing
  * set vposition on title page logo image explicitly to avoid page overrun?
  
  * allow pdf-page-layout (portrait || landscape) to be set in document
  * allow pdf-page-margin to be set in document
  * verse has problems with wrapping if line is long
  * allow links to be underlined (theme setting)
  * allow default kerning to be set using theme
  * keep line comment in front of callout number to aid copying?
  * table column header is wrapping by char (verify?)
  * implement margin collapsing (between blocks)
  * rework pull request for source line numbers (combine with restore conum logic if conums are enabled)
    - also combine with the preserve_space logic
  * allow front cover and back cover image to be defined in theme; document in theming-guide
  * should we shorten the keys to front-cover and back-cover (since image is implied?)
  * keep caption with table (check for sufficient space); only for top placement since bottom placement is much harder
  * allow valign to be set on image block (vertical center in page for things like slides)
  
negative.code: * large image runs into running footer (doesn't bottom margin need to be aligned with running footer height?)
  * should str_to_pt helper handle % and vw units?
  * allow style of visible link to be controlled by theme (similar to what we do in the manpage converter)
  * FILE ISSUE: should not wrap at formatting mark unless it is at a break opportunity
    - the problem here is that Prawn is allowing breaks at the boundaries of text fragments; it should only look at the contents
  * be more consistent with how examples are shown in theming-guide.adoc (do we show last segment only?)
  * allow top as alternative to margin_top for all elements on title page (#431)
  * swallowing exceptions! (any use of e.message in a string is dangerous)
  