Function extract_purpose_header_with_reader 

fn extract_purpose_header_with_reader<E, F>(
    path: &Path,
    rel_path: &str,
    config: &AtlasMapConfig,
    read_text: &mut F,
) -> Result<(Option<String>, Vec<String>), E>
where
    F: FnMut(&Path) -> Result<String, E>,

Extract a purpose header through a caller-owned bounded text reader.