be called as a function with @var{args} given as arguments. The
function should return a @var{split}.
+@cindex body split
For instance, the following function could be used to split based on the
body of the messages:
"string.group")))
@end lisp
+The @samp{" *nnmail incoming*"} is narrowed to the message in question
+when the @code{:} function is run.
+
@item
@code{(! @var{func} @var{split})}: If the split is a list, and the first
element is @code{!}, then SPLIT will be processed, and FUNC will be