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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 angle_to_cartesian(lon, lat):
 """Convert spherical coordinates to cartesian unit vectors."""

    theta = np.array(np.pi / 2. - lat)
    return np.vstack((np.sin(theta) * np.cos(lon),
                      np.sin(theta) * np.sin(lon),
                      np.cos(theta))).T