\[4(\boldsymbol{{\eta}}^{1}(T_{1})\cdot\boldsymbol{{\eta}}^{2}(T_{2})-%
\boldsymbol{{\eta}}^{2}(T_{1})\cdot\boldsymbol{{\eta}}^{1}(T_{2}))=|T_{1}\cap T%
_{2}|\pmod{2}.\]