See https://github.com/joshcheek/seeing_is_believing for more information
about what the gem does. Not that you must install the gem before you can
use this mode (gem install seeing_is_believing).

Once this mode is enabled you can use the following key bindings:
  <prefix> s    run seeing is believing with default settings
  <prefix> t    tag current line with the xmpfilter marker
  <prefix> x    run prefix args with xmpfilter compatability
  <prefix> c    clear seeing is believing output from a file

The default prefix is "C-c ?"