Function parse_cow 

pub fn parse_cow<'a, Iter, C>(
    input_file: impl AsRef<Path>,
    base_dir: impl AsRef<Path>,
    parameters: Iter,
) -> Result<String, Error>
where
    Iter: Iterator<Item = C>,
    C: Into<Cow<'a, str>>,

Same as parse, but the parameters iterator has an item of impl Into<Cow<str>>. This is so that an empty iterator can be passed to parse.