A trailing @code{?} after the stream index will allow the map to be
optional: if the map matches no streams the map will be ignored instead
of failing. Note the map will still fail if an invalid input file index
-is used; such as if the map refers to a non-existant input.
+is used; such as if the map refers to a non-existent input.
An alternative @var{[linklabel]} form will map outputs from complex filter
graphs (see the @option{-filter_complex} option) to the output file.