append( vertex ) {

		if ( this.head === null ) {

			this.head = vertex;

		} else {

			this.tail.next = vertex;

		}

		vertex.prev = this.tail;
		vertex.next = null; // the tail has no subsequent vertex

		this.tail = vertex;

		return this;

	}
Appends a vertex to this vertex list.

@private
@param {VertexNode} vertex - The vertex to append.
@return {VertexList} A reference to this vertex list.