docs: def prior_neighbor(C, alpha=0.001):
      r"""Neighbor prior of strength alpha for the given count matrix.
  
      Prior is defined by
          b_ij = alpha  if Z_ij+Z_ji > 0
          b_ij = 0      else
  
      Parameters
      ----------
      C : (M, M) scipy.sparse matrix
          Count matrix
      alpha : float (optional)
          Value of prior counts
  
      Returns
      -------
      B : (M, M) scipy.sparse matrix
          Prior count matrix
  
      """
      C_sym = C + C.transpose()
      C_sym = C_sym.tocoo()
      data = C_sym.data
      row = C_sym.row
      col = C_sym.col
  
      data_B = alpha * np.ones_like(data)
      B = coo_matrix((data_B, (row, col)))
      return B