code: def within(self, x, ctrs, kdtree=None):
          
  
          if kdtree is None:
              # If no KDTree is provided, execute a brute-force search
              # over all cubes.
              idxs = np.where(np.max(np.abs(ctrs - x), axis=1) <= self.hside)[0]
          else:
              # If a KDTree is provided, find all points within r (`hside`).
              idxs = kdtree.query_ball_point(x, self.hside, p=np.inf, eps=0)
  
          return idxs
docs: Checks which cubes `x` falls within. Uses a K-D Tree to
          perform the search if provided.