code: def within(self, x, ctrs, kdtree=None):
          
  
          if kdtree is None:
              # If no K-D Tree is provided, execute a brute-force
              # search over all balls.
              idxs = np.where(lalg.norm(ctrs - x, axis=1) <= self.radius)[0]
          else:
              # If a K-D Tree is provided, find all points within `self.radius`.
              idxs = kdtree.query_ball_point(x, self.radius, p=2.0, eps=0)
  
          return idxs
docs: Check which balls `x` falls within. Uses a K-D Tree to
          perform the search if provided.