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Solve the following problem using Python, implementing the functions described below, one line at a time
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Description:
 def skew(xi):
 """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. """

    S = np.array([[0, -xi[2], xi[1]],
                  [xi[2], 0, -xi[0]],
                  [-xi[1], xi[0], 0]])
    return S