public void addLLFuncExprForTopExpr(ExprNodeDesc topExpr, ExprNodeGenericFuncDesc llFuncExpr) {
    addLeadLagExpr(llFuncExpr);
    mapTopExprToLLFunExprs = mapTopExprToLLFunExprs == null ?
        new IdentityHashMap<ExprNodeDesc, List<ExprNodeGenericFuncDesc>>() : mapTopExprToLLFunExprs;
    List<ExprNodeGenericFuncDesc> funcList = mapTopExprToLLFunExprs.get(topExpr);
    if (funcList == null) {
      funcList = new ArrayList<ExprNodeGenericFuncDesc>();
      mapTopExprToLLFunExprs.put(topExpr, funcList);
    }
    funcList.add(llFuncExpr);
  }
map from the Select Expr Node to the LL Function invocations in it.
ql/src/java/org/apache/hadoop/hive/ql/parse/LeadLagInfo.java