First describe the connections
Goal: Identify vertices and edges before adding tree rules.
A graph describes objects and connections. An object is a vertex or node; a connection is an edge. Two nodes joined by one undirected edge form a graph. One isolated node with no edges also forms a graph.
An edge must have endpoints in the vertex set. A drawn line with no specified endpoints does not describe a valid edge by itself. Some mathematical conventions also allow an empty graph; we will use nonempty examples here.
A self-loop is an edge from a node to itself. This is a graph term, not a program loop. It is allowed in some graph models and excluded in a simple graph. Before classifying a drawing, state the graph model.
BST means Binary Search Tree. We will add the requirements behind those words one at a time.
Remember: A graph says what is connected; it need not be a tree.