Definition $1$: An evolutionary tree for a set of organisms is defined as below:

B$.$ X$1,$ if $.$

R$.,$ nonempty sets T$1$ and T$2$ are evolutionary trees such that and $,$ if $.$

Definition $2$: A Slist is defined below:

B$.$ x $,$ where $,$ the real numbers.

R$.$ where X and Y are SLists having the same number of elements, and the last number in X is less than the first number in Y$.$