* existing key-value pair has no effect.
 *
 * <p>Null keys and values are permitted (provided, of course, that the respective comparators
 * support them). All optional multimap methods are supported, and all returned views are
 * modifiable.
 *
 * <p>This class is not threadsafe when any concurrent operations update the multimap. Concurrent

## An app with callbacks

Let's see all this with an example.

Imagine you develop an app that allows creating invoices.

These invoices will have an `id`, `title` (optional), `customer`, and `total`.

The user of your API (an external developer) will create an invoice in your API with a POST request.

Then your API will (let's imagine):

```Python hl_lines="9  14  20  23  27"
{!> ../../docs_src/body_nested_models/tutorial007.py!}
```

////

/// info

Notice how `Offer` has a list of `Item`s, which in turn have an optional list of `Image`s

///

## Bodies of pure lists

To get a connection to a specific server, use `Manager.Connection(host)` to get a connection to the specified host.
From this connection individual requests can be made.

Each handler, with optional subroutes can be registered with the manager using 
`Manager.RegisterXHandler(handlerID, handler, subroutes...)`.

A `Handler()` function provides an HTTP handler, which should be hooked up to the appropriate route on the server.

/// tip | "Подсказка"

Pydantic, мощь которого используется в FastAPI для валидации и сериализации, имеет специальное поведение для `Optional` или `Union[Something, None]` без значения по умолчанию. Вы можете узнать об этом больше в документации Pydantic, раздел <a href="https://docs.pydantic.dev/latest/concepts/models/#required-optional-fields" class="external-link" target="_blank">Обязательные Опциональные поля</a>.

///

| com.squareup.okhttp3:mockwebserver3-junit4:5.0.0-alpha.2 | mockwebserver3.junit4 | Optional JUnit 4 integration.     |
| com.squareup.okhttp3:mockwebserver3-junit5:5.0.0-alpha.2 | mockwebserver3.junit5 | Optional JUnit 5 integration.     |
| com.squareup.okhttp3:mockwebserver:5.0.0-alpha.2         | okhttp3.mockwebserver | Obsolete. Depends on JUnit 4.     |

 *
 * ### Query
 *
 * The query is optional: it can be null, empty, or non-empty. For many HTTP URLs the query string
 * is subdivided into a collection of name-value parameters. This class offers methods to set the
 * query as the single string, or as individual name-value parameters. With name-value parameters
 * the values are optional and names may be repeated.
 *
 * ### Fragment
 *

        return parser.getAttributeValue(null, "implementation");
    }

    /**
     * Attempts to load the named implementation, uses default implementation if no name is given.
     *
     * @param name The optional implementation name
     * @param defaultClazz The default implementation type
     * @return Custom implementation type if one was given; otherwise default implementation type
     */

import org.codelibs.fess.mylasta.action.FessMessages;
import org.codelibs.fess.mylasta.direction.FessConfig;
import org.codelibs.fess.util.ComponentUtil;
import org.codelibs.fess.util.RenderDataUtil;
import org.dbflute.optional.OptionalEntity;
import org.lastaflute.web.Execute;
import org.lastaflute.web.response.HtmlResponse;
import org.lastaflute.web.response.render.RenderData;
import org.lastaflute.web.ruts.process.ActionRuntime;

 * Updating the table can change the contents of those collections, and updating the collections
 * will change the table.
 *
 * <p>All methods that modify the table are optional, and the views returned by the table may or may
 * not be modifiable. When modification isn't supported, those methods will throw an {@link
 * UnsupportedOperationException}.
 *
 * <h3>Implementations</h3>
 *
 * <ul>