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:  ** allow image to span width of page (role=canvas, role=canvas-x or role=canvas-y); if role is canvas or canvas-y, then it does not consume height
   ** partially addressed by vw units
  * FILE ISSUE: when split source listing, add top padding to bounding box (or is it the line metrics top?)
   ** perhaps this has to do with the a miscalculation in dry run when not starting from same y position?
   ** separate theme control for listing vs literal block (and maybe source too)
  * support URL images in running content (need to delegate to resolve_image_path)
  * add feature to number bullets according to section number (needed for OpenDevise agreements)
   ** allow ordered list marker to be prefixed by section number (a global setting?)
  
negative.code: * don't create title page for article doctype (#95, #105)
   ** only create title page if doctype=book
  * allow character spacing to be controlled by theme
  * allow pdf-page-margin to be set in document
   ** intended primarily for image slideshows
   ** this is slighly more complicated now that we have mirror margins; perhaps can't set those from document?
  