docs: def poisson_dist(p1, p2):
      """
      Calculates the Poisson distance between two vectors.
  
      p1 can be a sparse matrix, while p2 has to be a dense matrix.
      """
      # ugh...
      p1_ = p1 + eps
      p2_ = p2 + eps
      return np.dot(p1_-p2_, np.log(p1_/p2_))