PostOrderNode(T root, Iterator<T> childIterator) {
      this.root = checkNotNull(root);
      this.childIterator = checkNotNull(childIterator);
    }
  }

  private final class PostOrderIterator extends AbstractIterator<T> {
    private final ArrayDeque<PostOrderNode<T>> stack;

    PostOrderIterator(T root) {
      this.stack = new ArrayDeque<>();
      stack.addLast(expand(root));
    }

    @Override

        @Override
        Map<K, Collection<V>> map() {
          return AsMap.this;
        }

        @Override
        public Iterator<Entry<K, Collection<V>>> iterator() {
          return new AbstractIterator<Entry<K, Collection<V>>>() {
            final Iterator<Entry<K, Collection<V>>> backingIterator =
                unfiltered.asMap().entrySet().iterator();

            @Override

      @Override
      public boolean isEmpty() {
        return set1.isEmpty() && set2.isEmpty();
      }

      @Override
      public UnmodifiableIterator<E> iterator() {
        return new AbstractIterator<E>() {
          final Iterator<? extends E> itr1 = set1.iterator();
          final Iterator<? extends E> itr2 = set2.iterator();

          @Override
          protected @Nullable E computeNext() {

      }

      @Override
      public boolean retainAll(Collection<?> c) {
        return removeFromColumnIf(not(in(c)));
      }
    }

    private final class EntrySetIterator extends AbstractIterator<Entry<R, V>> {
      final Iterator<Entry<R, Map<C, V>>> iterator = backingMap.entrySet().iterator();

      @Override
      protected @Nullable Entry<R, V> computeNext() {
        while (iterator.hasNext()) {

    PostOrderNode(T root, Iterator<T> childIterator) {
      this.root = checkNotNull(root);
      this.childIterator = checkNotNull(childIterator);
    }
  }

  private final class PostOrderIterator extends AbstractIterator<T> {
    private final ArrayDeque<PostOrderNode<T>> stack;

    PostOrderIterator(T root) {
      this.stack = new ArrayDeque<>();
      stack.addLast(expand(root));
    }

    @Override

        @Override
        Map<K, Collection<V>> map() {
          return AsMap.this;
        }

        @Override
        public Iterator<Entry<K, Collection<V>>> iterator() {
          return new AbstractIterator<Entry<K, Collection<V>>>() {
            final Iterator<Entry<K, Collection<V>>> backingIterator =
                unfiltered.asMap().entrySet().iterator();

            @Override

    Iterator<C> merged =
        mergeSorted(
            transform(backingMap.values(), (Map<C, V> input) -> input.keySet().iterator()),
            comparator);

    return new AbstractIterator<C>() {
      @Nullable C lastValue;

      @Override
      protected @Nullable C computeNext() {
        while (merged.hasNext()) {
          C next = merged.next();

import com.google.common.annotations.J2ktIncompatible;
import com.google.common.base.Ascii;
import com.google.common.base.Optional;
import com.google.common.base.Splitter;
import com.google.common.collect.AbstractIterator;
import com.google.common.collect.ImmutableList;
import com.google.errorprone.annotations.CanIgnoreReturnValue;
import com.google.errorprone.annotations.MustBeClosed;
import java.io.BufferedReader;

import com.google.common.annotations.J2ktIncompatible;
import com.google.common.base.Ascii;
import com.google.common.base.Optional;
import com.google.common.base.Splitter;
import com.google.common.collect.AbstractIterator;
import com.google.common.collect.ImmutableList;
import com.google.errorprone.annotations.CanIgnoreReturnValue;
import com.google.errorprone.annotations.MustBeClosed;
import java.io.BufferedReader;

    Iterator<C> merged =
        mergeSorted(
            transform(backingMap.values(), (Map<C, V> input) -> input.keySet().iterator()),
            comparator);

    return new AbstractIterator<C>() {
      @Nullable C lastValue;

      @Override
      protected @Nullable C computeNext() {
        while (merged.hasNext()) {
          C next = merged.next();