@SuppressWarnings("unchecked")
  public void setNodeTypes(Class<? extends Node>... nodeTypes) {
    this.nodeTypes.addAll(Arrays.asList(nodeTypes));
  }
Constructor with specified number of ancestor levels to keep in the
operator stack during dispatching.

@param disp Dispatcher to call for each op encountered
@param numLevels Number of ancestor levels
ql/src/java/org/apache/hadoop/hive/ql/lib/LevelOrderWalker.java