To create the callback *path operation* use the same `APIRouter` you created above.

It should look just like a normal FastAPI *path operation*:

* It should probably have a declaration of the body it should receive, e.g. `body: InvoiceEvent`.
* And it could also have a declaration of the response it should return, e.g. `response_model=InvoiceEventReceived`.

```Python hl_lines="16-18  21-22  28-32"

            isNullable,
            type(sourceName),
            variance,
            typeArguments.map { apiTypeUsageFor(it.binaryName, variance = it.variance, typeArguments = it.typeArguments) },
            bounds.map { apiTypeUsageFor(it.binaryName, variance = it.variance, typeArguments = it.typeArguments) }
        )
    }


internal
val ApiTypeUsage.isStarProjectionTypeUsage

The result of calling it is something that can be encoded with the Python standard <a href="https://docs.python.org/3/library/json.html#json.dumps" class="external-link" target="_blank">`json.dumps()`</a>.

It doesn't return a large `str` containing the data in JSON format (as a string). It returns a Python standard data structure (e.g. a `dict`) with values and sub-values that are all compatible with JSON.

!!! note

    val response1Body =
      client.newCall(request).execute().use {
        if (!it.isSuccessful) throw IOException("Unexpected code $it")

        println("Response 1 response:          $it")
        println("Response 1 cache response:    ${it.cacheResponse}")
        println("Response 1 network response:  ${it.networkResponse}")
        return@use it.body.string()
      }

    val response2Body =

    ```

!!! note
    A path parameter is always required as it has to be part of the path.

    So, you should declare it with `...` to mark it as required.

    Nevertheless, even if you declared it with `None` or set a default value, it would not affect anything, it would still be always required.

## Order the parameters as you need

!!! tip

            collector.typesToShorten.distinctBy { it.element }.map { TypeToShortenInfo(it.element.createSmartPointer(), it.shortenedRef) },
            collector.qualifiersToShorten.distinctBy { it.element }.map { QualifierToShortenInfo(it.element.createSmartPointer(), it.shortenedRef) },
            collector.labelsToShorten.distinctBy { it.element }.map { ThisLabelToShortenInfo(it.element.createSmartPointer()) },

     * @param pairs
     * @param type
     */
    public static void remove ( List<AvPair> pairs, int type ) {
        Iterator<AvPair> it = pairs.iterator();
        while ( it.hasNext() ) {
            AvPair p = it.next();
            if ( p.getType() == type ) {
                it.remove();
            }
        }
    }


    /**
     * Replace all occurances of the given type
     * 

        By asking questions in a structured way (following this) it will be much easier to help you.

        And there's a high chance that you will find the solution along the way and you won't even have to submit it and wait for an answer. 😎

When you request a URL with OkHttp, here's what it does:

 1. It uses the URL and configured OkHttpClient to create an **address**. This address specifies how we'll connect to the webserver.
 2. It attempts to retrieve a connection with that address from the **connection pool**.

    for (auto it : op_it->second.input_tensor_id) {
      auto count_it = result.tensor_usage_counts.find(it);
      if (count_it != result.tensor_usage_counts.end()) {
        count_it->second++;
      } else {
        result.tensor_usage_counts[it] = 1;
        if (tensor_tape.find(it) != tensor_tape.end()) {
          tensor_stack.push_back(it);
        }
      }
    }