I am always wondering if I defined the quantum numbers of the fermion site basis in the following way, would there be a negative Ndown number (for example, with N=0, Nup=1)?

<QUANTUMNUMBER name="N" min="0" max="2"/>

<QUANTUMNUMBER name="Nup" min="0" max="1"/>

<QUANTUMNUMBER name="Ndown" min="N-Nup" max="N-Nup"/>
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