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Solve the following problem using Python, implementing the functions described below, one line at a time
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Description:
 def _build_paths(self, name, spec_path_lists, exists):
 """ Given an environment variable name and specified paths, return a pathsep-separated string of paths containing unique, extant, directories from those paths and from the environment variable. Raise an error if no paths are resolved. """

        # flatten spec_path_lists
        spec_paths = itertools.chain.from_iterable(spec_path_lists)
        env_paths = safe_env.get(name, '').split(os.pathsep)
        paths = itertools.chain(spec_paths, env_paths)
        extant_paths = list(filter(os.path.isdir, paths)) if exists else paths
        if not extant_paths:
            msg = "%s environment variable is empty" % name.upper()
            raise distutils.errors.DistutilsPlatformError(msg)
        unique_paths = self._unique_everseen(extant_paths)
        return os.pathsep.join(unique_paths)