void OnMouseDown() {
            pressed = true;
            if (!isClone) {
                if (transform.parent.name.Contains("AND")) {
                    clone = Instantiate(Resources.Load("FB/FunctionBlock_AND")) as GameObject;
                      ...
                }
            }
            screenPoint = Camera.main.WorldToScreenPoint (gameObject.transform.position);

            offset = gameObject.transform.position - Camera.main.ScreenToWorldPoint (new Vector3 (Input.mousePosition.x, Input.mousePosition.y, screenPoint.z));
        }