code: def skew(xi):
      
      S = np.array([[0, -xi[2], xi[1]],
                    [xi[2], 0, -xi[0]],
                    [-xi[1], xi[0], 0]])
      return S
docs: Return the skew-symmetric matrix that can be used to calculate
      cross-products with vector xi.
  
      Multiplying this matrix by a vector `v` gives the same result
      as `xi x v`.
  
      Parameters
      ----------
      xi : :obj:`numpy.ndarray` of float
          A 3-entry vector.
  
      Returns
      -------
      :obj:`numpy.ndarray` of float
          The 3x3 skew-symmetric cross product matrix for the vector.