code: def second_order_moments_u(adata):
      
      if 'neighbors' not in adata.uns:
          raise ValueError('You need to run `pp.neighbors` first to compute a neighborhood graph.')
  
      connectivities = get_connectivities(adata)
      u = csr_matrix(adata.layers['unspliced'])
      Muu = csr_matrix.dot(connectivities, u.multiply(u)).astype(np.float32).A
  
      return Muu
docs: Computes second order moments for stochastic velocity estimation.
  
      Arguments
      ---------
      adata: `AnnData`
          Annotated data matrix.
  
      Returns
      -------
      Muu: Second order moments for unspliced abundances