拾取一点和拾取另一点时, 实现线段跟随的效果, 实现右键，空格，ESC都可以退出效果

        /// <summary>
        /// 交互得到另一点  //  houlinbo QQ420021327
        /// </summary>
        /// <returns></returns>
        public static PromptPointResult SelectOtherPt(string strMsg, Point3d pt)
        {
            Document doc = Application.DocumentManager.MdiActiveDocument;
            Editor ed = doc.Editor;
            PromptPointOptions prpointoptions = new PromptPointOptions(strMsg);
            prpointoptions.BasePoint = pt;          // 
            prpointoptions.UseBasePoint = true;  //可实现线段跟随的效果
            prpointoptions.AllowNone = true;      //右键，空格，ESC都可以退出
            PromptPointResult prPointRes = ed.GetPoint(prpointoptions);
            return prPointRes;
        }