public new ExpressValue this[string pKey] {
     get {
         if (base[pKey] != null)
         {
             ExpressValue val = base[pKey] as ExpressValue;
             return(val);
         }
         else
         {
             return(null);
         }
     }
 }
 public ExpressValue Add(ExpressValue pExpress)
 {
     this.Add(pExpress.Name, pExpress);
     return(pExpress);
 }