Binary tree child slot
= Binary tree child slot
{title2=$(L,R)$}
One of the two distinguished positions, left or right, at a <vertex> of a <binary tree>. Each <binary tree child slot> is occupied by a child and its <subtree>, or is empty. A reference representation can use `null` for an empty <binary tree child slot>. Two slots are capacity for at most two <children in a rooted tree>, rather than a requirement for two actual <vertices>.