private void addLeadLagExpr(ExprNodeGenericFuncDesc llFunc) {
    leadLagExprs = leadLagExprs == null ? new ArrayList<ExprNodeGenericFuncDesc>() : leadLagExprs;
    leadLagExprs.add(llFunc);
  }
map from the Select Expr Node to the LL Function invocations in it.
ql/src/java/org/apache/hadoop/hive/ql/parse/LeadLagInfo.java