or nil. Nil means use the default sort order.
:search-type Specify whether to run the search in search-mode,
tree mode or unthreaded mode. Set to `tree' to
- specify tree mode, 'unthreaded to specify
+ specify tree mode, \\='unthreaded to specify
unthreaded mode, and set to nil (or anything
except tree and unthreaded) to specify search
mode.