The string search is case insensitive.
If
.Ar x
is ommitted, it will default to the
is omitted, it will default to the
.Dq Li Subject
header field.
The form
.Ar file
or the standard input if the
argument is ommitted,
argument is omitted,
applies a configurable line numbering filter operation and writes the result
to the standard output.
.Pp