def select_known_backend(backend):
                """
                Loads a specific threading layer backend based on string
                """
                lib = None
                if backend.startswith("tbb"):
                    try:
                        # check if TBB is present and compatible
                        _check_tbb_version_compatible()
                        # now try and load the backend
                        from numba.np.ufunc import tbbpool as lib
                    except ImportError:
                        pass
                elif backend.startswith("omp"):
                    # TODO: Check that if MKL is present that it is a version
                    # that understands GNU OMP might be present
                    try:
                        from numba.np.ufunc import omppool as lib
                    except ImportError:
                        pass
                elif backend.startswith("workqueue"):
                    from numba.np.ufunc import workqueue as lib
                else:
                    msg = "Unknown value specified for threading layer: %s"
                    raise ValueError(msg % backend)
                return lib

                Loads a specific threading layer backend based on string
                
numba/np/ufunc/parallel.py