The default value of variables defined with define_variable is remembered for
use by the help system; however if the variable is changed the default value is
incorrectly reported. This may only apply to certain types of variables.

1. Change one of the default MIME type external handlers:
define_mime_type_external_handler("application/pdf", "xpdf");
2. C-h v mime_type_external_handlers
3. Notice that the default listed for /^application\/pdf$/ is xpdf when it
should be evince.