So, what we will do next is add the code to document how that *external API* should look like to receive the callback from *your API*.

That documentation will show up in the Swagger UI at `/docs` in your API, and it will let external developers know how to build the *external API*.

This example doesn't implement the callback itself (that could be just a line of code), only the documentation part.

/// tip

        return systemProperties;
    }

    /**
     * Sets the system properties to use for interpolation and profile activation. The system properties are collected
     * from the runtime environment like {@link System#getProperties()} and environment variables.
     *
     * @param systemProperties The system properties, may be {@code null}.
     * @return This context, never {@code null}.
     */

     * environment like {@link System#getProperties()} and environment variables.
     *
     * @return The system properties, never {@code null}.
     */
    Properties getSystemProperties();

    /**
     * Sets the system properties to use for interpolation. The system properties are collected from the runtime
     * environment like {@link System#getProperties()} and environment variables.
     *

     *            {@link org.apache.maven.building.Problem.Severity#ERROR}.
     * @param source A hint about the source of the problem like a file path, may be {@code null}.
     * @param lineNumber The one-based index of the line containing the problem or {@code -1} if unknown.
     * @param columnNumber The one-based index of the column containing the problem or {@code -1} if unknown.

     *            {@link ModelProblem.Severity#ERROR}.
     * @param version The version since the problem is relevant
     * @param source A hint about the source of the problem like a file path, may be {@code null}.
     * @param lineNumber The one-based index of the line containing the problem or {@code -1} if unknown.
     * @param columnNumber The one-based index of the column containing the problem or {@code -1} if unknown.

   */
  public static List<String> readLines(Readable r) throws IOException {
    List<String> result = new ArrayList<>();
    LineReader lineReader = new LineReader(r);
    String line;
    while ((line = lineReader.readLine()) != null) {
      result.add(line);
    }
    return result;
  }

  /**
   * Streams lines from a {@link Readable} object, stopping when the processor returns {@code false}

		}
	}
	in.Push(NewTokenizer(name, fd, fd))
}

// #line processing.
func (in *Input) line() {
	// Only need to handle Plan 9 format: #line 337 "filename"
	tok := in.Stack.Next()
	if tok != scanner.Int {
		in.expectText("expected line number after #line")
	}
	line, err := strconv.Atoi(in.Stack.Text())
	if err != nil {
		in.Error("error parsing #line (cannot happen):", err)
	}
	tok = in.Stack.Next()

# nightly job. update_version.py affects TF_VER_SUFFIX, TF_VER_PYTHON, and
# TF_VER_FULL.

# Note: in awk, the command '/search/ {commands}' applies the commands to any line that
# matches the /search/ regular expression. "print $N" prints the Nth "field",
# where fields are strings separated by whitespace.
export TF_VER_MAJOR=$(awk '/#define TF_MAJOR_VERSION/ {print $3}' tensorflow/core/public/version.h)

#             ; freeGenList = list:{ [env or config or label or message or html] }
#             ; propertiesHtmlList = list:{ [env or config or label or message] }
#         }
#     }
# }
#
# *The line that starts with '#' means comment-out.
#
map:{
    # your service name, camel case, initial uncapitalised
    ; serviceName = fess

    # package for your domain name, e.g. com.example

///

## Response Status Code

You can define the (HTTP) `status_code` to be used in the response of your *path operation*.

You can pass directly the `int` code, like `404`.

But if you don't remember what each number code is for, you can use the shortcut constants in `status`:

//// tab | Python 3.10+

```Python hl_lines="1  15"