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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 tropocollagen( cls, aa=28, major_radius=5.0, major_pitch=85.0, auto_build=True):
 """Creates a model of a collagen triple helix. Parameters aa : int, optional Number of amino acids per minor helix. major_radius : float, optional Radius of super helix. major_pitch : float, optional Pitch of super helix. auto_build : bool, optional If `True`, the model will be built as part of instantiation. """

        instance = cls.from_parameters(
            n=3, aa=aa, major_radius=major_radius, major_pitch=major_pitch,
            phi_c_alpha=0.0, minor_helix_type='collagen', auto_build=False)
        instance.major_handedness = ['r'] * 3
        # default z-shifts taken from rise_per_residue of collagen helix
        rpr_collagen = _helix_parameters['collagen'][1]
        instance.z_shifts = [-rpr_collagen * 2, -rpr_collagen, 0.0]
        instance.minor_repeats = [None] * 3
        if auto_build:
            instance.build()
        return instance