public Entry search_level(int slevel, String myid) {
     if (slevel > level) {
         return null;
     }
     if (slevel < 0) { // be defensive
         System.err.println("oops slevel<0 " + slevel);
         System.exit(1);
     }
     // go directly to right block:
     ArrayList<Entry> block = st.elementAt(level-slevel);
     // now search that block:
     for( Entry p : block) {
         if( p.getName().equals(myid) ){
             return p;
         }
     }
     return null;
 }