code: def parse_references(cls, filename):
          
          references = set()
          for line in open(filename):
              matched = cls.RE_REF.match(line)
              if matched:
                  reference = matched.group('path')
                  reference_base = os.path.splitext(reference)[0]
                  references.add(reference_base)
          return references
docs: Read filename line by line searching for pattern:
  
          -r file.in
          or
          --requirement file.in
  
          return set of matched file names without extension.
          E.g. ['file']