@SuppressWarnings("unchecked")
  protected void addAllParents(Node nd) {
    Operator<? extends OperatorDesc> op = (Operator<? extends OperatorDesc>) nd;
    toWalk.removeAll(op.getParentOperators());
    toWalk.addAll(0, op.getParentOperators());
  }
Constructor.

@param disp
dispatcher to call for each op encountered
ql/src/java/org/apache/hadoop/hive/ql/lib/ForwardWalker.java