private boolean checkReferences(List<ExprNodeDesc> sources, Set<String> funcOutputs) {
      Set<String> ref = new HashSet<String>();
      for (ExprNodeDesc source : sources) {
        if (!checkReferences(source, funcOutputs, ref)) {
          return false;
        }
      }
      return true;
    }
merges SEL-SEL or FIL-FIL into single operator
ql/src/java/org/apache/hadoop/hive/ql/optimizer/NonBlockingOpDeDupProc.java