function(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;
    }
function(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;
    }
function ( 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 the target vertex
Inserts a vertex before the target vertex