code: def _twosided_zerolag(data, zerolag):
      
      res = twosided(np.insert(data, 0, zerolag))
      return res
docs: Build a symmetric vector out of stricly positive lag vector and zero-lag
  
      .. doctest::
  
          >>> data = [3,2,1]
          >>> zerolag = 4
          >>> twosided_zerolag(data, zerolag)
          array([1, 2, 3, 4, 3, 2, 1])
  
      .. seealso:: Same behaviour as :func:`twosided_zerolag`