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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 subtract_vectors(a, b, periodic):
    '''Returns the difference of the points vec_a - vec_b subject 
       to the periodic boundary conditions.

    '''
    r = a - b
    delta = np.abs(r)
    sign = np.sign(r)
    return np.where(delta > 0.5 * periodic, sign * (periodic - delta), r)