<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:
 def _bnd(self, xloc, dist, length, cache):
 """ boundary function. Example: [[[0. 0. 0.] [0. 0. 0.]] <BLANKLINE> [[2. 2. 2.] [2. 2. 2.]]] """

        lower, upper = evaluation.evaluate_bound(
            dist, xloc.reshape(1, -1))
        lower = lower.reshape(length, -1)
        upper = upper.reshape(length, -1)
        assert lower.shape == xloc.shape, (lower.shape, xloc.shape)
        assert upper.shape == xloc.shape
        return lower, upper