new_contents: import numpy as np
  from scipy.stats import multinomial
  
  def BRIE_base_lik(psi, counts, lengths):
      """Base likelihood function of BRIE model
      """
      size_vect = np.array([psi, (1 - psi), 1]) * lengths
      prob_vect = size_vect / np.sum(size_vect)
      
      rv = multinomial(np.sum(counts), prob_vect)
      return rv.pmf(counts)
subject: add base likelihood