const ForwardOperation& op,
    std::unique_ptr<GradientFunction>* gradient_function) const {
  auto iter = registry_.find(op.op_name);
  if (iter == registry_.end()) {
    return absl::NotFoundError(
        absl::StrCat("No gradient defined for op: ", op.op_name, "."));
  }
  gradient_function->reset(iter->second(op));
  return absl::OkStatus();
}

TapeTensor::TapeTensor(AbstractTensorHandle* handle) : handle_(handle) {

        StringBuilder commandWithRetry = new StringBuilder("for iter in " + retrySequence + "; do \n");
        commandWithRetry.append(indent).append("  ").append(command).append(" && \n");
        commandWithRetry.append(indent).append("  exit_code=0 && break || \n");
        commandWithRetry.append(indent);
        commandWithRetry.append("    exit_code=$? && echo \"").append(name).append(" error: retry $iter in 10s\" && sleep 10; \n");

    List<Integer> result = Lists.newArrayListWithCapacity(initial.size());
    for (Iterator<Integer> iter = q.iterator(); iter.hasNext(); ) {
      Integer value = iter.next();
      result.add(value);
      if (value == 8) {
        iter.remove();
      }
    }
    assertIntact(q);
    assertThat(result).containsExactly(1, 15, 13, 8, 14);
  }

  /**

                                    " into function '", fn_name, "'");

    input_tensors->emplace_back(node, idx);

    const auto& iter = input_nodes->find(node);
    if (iter == input_nodes->end()) {
      input_nodes->insert({node, {idx}});
    } else {
      auto& indices = iter->second;
      if (std::find(indices.begin(), indices.end(), idx) != indices.end()) {
        return InvalidArgument("TF_Output ", node->name(), ":", idx,

        StringBuilder buffer = new StringBuilder("ManagedVersionMap (" + size() + " entries)\n");
        Iterator<String> iter = keySet().iterator();
        while (iter.hasNext()) {
            String key = iter.next();
            buffer.append(key).append('=').append(get(key));
            if (iter.hasNext()) {
                buffer.append('\n');
            }
        }
        return buffer.toString();
    }

    assertEquals(1, (int) iter.next());
    assertTrue(iter.hasNext());
    assertEquals(2, (int) iter.next());
    assertTrue(iter.hasNext());
    assertEquals(3, (int) iter.next());
    assertFalse(iter.hasNext());
  }

  public void testAsEnumerationEmpty() {
    Iterator<Integer> iter = emptyIterator();
    Enumeration<Integer> enumer = Iterators.asEnumeration(iter);

        StringBuilder ret = new StringBuilder().append(proto).append(":").append(server).append("[").append(endpoint);
        if (options != null) {
            final Iterator iter = options.keySet().iterator();
            while (iter.hasNext()) {
                final Object key = iter.next();
                final Object val = options.get(key);
                ret.append(",").append(key).append("=").append(val);
            }
        }

        assertNotNull(set, "No artifacts");
        assertFalse(set.isEmpty(), "No Artifacts");
        Iterator iter = set.iterator();
        assertTrue(set.size() == 4, "Set size should be 4, is " + set.size());

        while (iter.hasNext()) {
            Artifact artifact = (Artifact) iter.next();
            System.out.println("Artifact: " + artifact.getDependencyConflictId() + " " + artifact.getVersion()

    assertEquals(1, (int) iter.next());
    assertTrue(iter.hasNext());
    assertEquals(2, (int) iter.next());
    assertTrue(iter.hasNext());
    assertEquals(3, (int) iter.next());
    assertFalse(iter.hasNext());
  }

  public void testAsEnumerationEmpty() {
    Iterator<Integer> iter = emptyIterator();
    Enumeration<Integer> enumer = Iterators.asEnumeration(iter);

	if elem == nil {
		return nil, nil
	}
	return iterToValue(elem.Iter)
}

func iterToValue(iter simdjson.Iter) (*sql.Value, error) {
	switch iter.Type() {
	case simdjson.TypeString:
		v, err := iter.String()
		if err != nil {
			return nil, err
		}
		return sql.FromString(v), nil
	case simdjson.TypeFloat:
		v, err := iter.Float()
		if err != nil {
			return nil, err
		}
		return sql.FromFloat(v), nil