def _init_composite_beta(model, robo, j):
    """
    Initialise the composite beta wrench of link j.

    Args:
        model: An instance of DynModel
        robo: An instance of Robot
        j: link number

    Returns:
        An instance of DynModel that contains all the new values.
    """
    model.composite_betas[j] = model.betas[j]
    return model