<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:
 def n_to_pf(L, n, R):
 """Returns the packing fraction for a number of non-intersecting spheres. Parameters L: float array, shape (d,) System lengths. n: integer Number of spheres. R: float Sphere radius. Returns ------- pf: float Fraction of space occupied by the spheres. """

    dim = L.shape[0]
    return (n * sphere_volume(R=R, n=dim)) / np.product(L)