public void reset() {
    currTask = null;
    currTopOp = null;
    currUnionOp = null;
    currAliasId = null;
  }
The context is reused across the rules. Reset so the following info is not
incorrectly carried over to the following optimizations starting with the new TS.
ql/src/java/org/apache/hadoop/hive/ql/optimizer/GenMRProcContext.java