The quartet distance between two binary trees $T_1$ and $T_2$
is the number of quartets consistent with only one tree.
Formally, $d_\textrm{q}(T_1, T_2) = 2\binom{n}{4} - 2q(T_1, T_2)$, where $q(T_1, T_2)$ is
the number of quartets shared by $T_1$ and $T_2$.
