def _filespec_for_excludes(cls, raw_excludes):
    excluded_patterns = []
    for exclude in cls.legacy_globs_class.process_raw_excludes(raw_excludes):
      exclude_filespecs = cls.from_sources_field(exclude).filespecs
      if exclude_filespecs.get('exclude', []):
        raise ValueError('Nested excludes are not supported: got {}'.format(raw_excludes))
      excluded_patterns.extend(exclude_filespecs.get('globs', []))
    return cls.legacy_globs_class.to_filespec(excluded_patterns)