docs: def _generate_a_indptr(num_states, s_indices, out):
      """
      Generate `a_indptr`; stored in `out`. `s_indices` is assumed to be
      in sorted order.
  
      Parameters
      ----------
      num_states : scalar(int)
  
      s_indices : ndarray(int, ndim=1)
  
      out : ndarray(int, ndim=1)
          Length must be num_states+1.
  
      """
      idx = 0
      out[0] = 0
      for s in range(num_states-1):
          while(s_indices[idx] == s):
              idx += 1
          out[s+1] = idx
      out[num_states] = len(s_indices)