private boolean shouldPrint(Explain exp, Object val) {
    if (exp.displayOnlyOnTrue() && (val instanceof Boolean) & !((Boolean)val)) {
      return false;
    }
    return true;
  }
use case: we want to print the object in explain only if it is true
how to do : print it unless the following 3 are all true:
1. displayOnlyOnTrue tag is on
2. object is boolean
3. object is false
@param exp
@param val
@return
ql/src/java/org/apache/hadoop/hive/ql/exec/ExplainTask.java