<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:
 def bounding_sphere(self):
 """ A minimum volume bounding sphere for the current mesh. Note that the Sphere primitive returned has an unpadded, exact sphere_radius so while the distance of every vertex of the current mesh from sphere_center will be less than sphere_radius, the faceted sphere primitive may not contain every vertex Returns -------- minball: trimesh.primitives.Sphere Sphere primitive containing current mesh """

        from . import primitives, nsphere
        center, radius = nsphere.minimum_nsphere(self)
        minball = primitives.Sphere(center=center,
                                    radius=radius,
                                    mutable=False)
        return minball