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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_from_requirements(cls, req_spec):
 """ Build a working set from a requirement spec. Rewrites sys.path. """

        # try it without defaults already on sys.path
        # by starting with an empty path
        ws = cls([])
        reqs = parse_requirements(req_spec)
        dists = ws.resolve(reqs, Environment())
        for dist in dists:
            ws.add(dist)

        # add any missing entries from sys.path
        for entry in sys.path:
            if entry not in ws.entries:
                ws.add_entry(entry)

        # then copy back to sys.path
        sys.path[:] = ws.entries
        return ws