def torsion(a, b, c):
    """
    Find the torsion angle between planes ab and bc.
    
    Arguments:
        
        *a,b,c*
            Vector instances.
    
    Returns:
        The torsion angle in radians
    """
    n1 = a.cross(b)
    n2 = b.cross(c)
    return n1.arg(n2)