code: def ksum(p, K=2):
      
      # Don't override the input data.
      q = p.copy()
      distill(q, K - 1)
      return numpy.sum(q[:-1], axis=0) + q[-1]
docs: From
  
      T. Ogita, S.M. Rump, and S. Oishi.
      Accurate Sum and Dot Product,
      SIAM J. Sci. Comput., 26(6), 1955–1988 (34 pages).
      <https://doi.org/10.1137/030601818>.
  
      Algorithm 4.8. Summation as in K-fold precision by (K−1)-fold error-free
      vector transformation.