def append_real_ensemble(member, x):
    """
    Append an element to the an existing ensemble

    The uncertain number ``x`` must be elementary and have the 
    same number of degrees of freedom as other members 
    of the ensemble (not checked). 
    
    """
    assert x.df == member._node.df
    assert x.is_elementary
    assert x._node.independent == False
    member._node.ensemble.add(x._node.uid)
    x._node.ensemble = member._node.ensemble
