insertBefore( target, vertex ) {

		vertex.prev = target.prev;
		vertex.next = target;

		if ( vertex.prev === null ) {

			this.head = vertex;

		} else {

			vertex.prev.next = vertex;

		}

		target.prev = vertex;

		return this;

	}
Inserts a vertex before a target vertex.

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