A quartet, denoted $\{a, b\} \mid \{c, d\}$, represents the relative order of four taxa$a$, $b$, $c$, and $d$
in a phylogeny. Specifically, it means that there exists an edge of the tree whose removal
separates the tree into one component containing $a$ and $b$ and another containing $c$ and $d$.

For example, note that $\{a, b\} \mid \{c, d\}$ is the only allowable quartet in the
tree below.