If $T_1$ and $T_2$ are both unrooted binary trees on the same $n$ taxa, then we now let $q(T_1, T_2)$
denote the number of inferred quartets that are common to both trees.
The quartet distance between $T_1$ and $T_2$, $d_{\textrm{q}}(T_1, T_2)$ is the number of
quartets that are only inferred from one of the trees. More precisely, $d_\textrm{q}(T_1, T_2) = q(T_1) + q(T_2) - 2q(T_1, T_2)$.

Given: A list containing $n$ taxa ($n \leq 2000$) and two unrooted binary trees $T_1$ and $T_2$
on the given taxa. Both $T_1$ and $T_2$ are given in Newick format.

Return: The quartet distance $d_\textrm{q}(T_1, T_2)$.