code: def within(self, x, j=None):
          
  
          # Loop through distance calculations if there aren't too many.
          idxs = np.where([self.ells[i].contains(x) if i != j else True
                           for i in range(self.nells)])[0]
  
          return idxs
docs: Checks which ellipsoid(s) `x` falls within, skipping the `j`-th
          ellipsoid.