docs: def jtosparse(j):
      """
      Generate sparse matrix coordinates from 3-D Jacobian.
      """
      data = j.flatten().tolist()
      nobs, nf, nargs = j.shape
      indices = zip(*[(r, c) for n in xrange(nobs)
                      for r in xrange(n * nf, (n + 1) * nf)
                      for c in xrange(n * nargs, (n + 1) * nargs)])
      return csr_matrix((data, indices), shape=(nobs * nf, nobs * nargs))