* multiple variants of ASN.1, including:
 *
 *  * DER: Distinguished Encoding Rules. This further constrains ASN.1 for deterministic encoding.
 *  * BER: Basic Encoding Rules.
 *
 * This class was implemented according to the [X.690 spec][[x690]], and under the advice of
 * [Lets Encrypt's ASN.1 and DER][asn1_and_der] guide.
 *
 * [x690]: https://www.itu.int/rec/T-REC-X.690

 *
 * The end of the range is not encoded, but can be inferred by looking at the start of the range
 * that follows.
 *
 * b1
 * --
 *
 * This is either a mapping decision or the length of the mapped output, according to this table:
 *
 * ```
 *  0..63 : Length of the UTF-16 sequence that this range maps to. The offset is b2b3.
 * 64..79 : Offset by a fixed negative offset. The bottom 4 bits of b1 are the top 4 bits of the offset.

/*
 * Licensed to the Apache Software Foundation (ASF) under one
 * or more contributor license agreements.  See the NOTICE file
 * distributed with this work for additional information
 * regarding copyright ownership.  The ASF licenses this file
 * to you under the Apache License, Version 2.0 (the
 * "License"); you may not use this file except in compliance
 * with the License.  You may obtain a copy of the License at
 *

/*
 * Licensed to the Apache Software Foundation (ASF) under one
 * or more contributor license agreements.  See the NOTICE file
 * distributed with this work for additional information
 * regarding copyright ownership.  The ASF licenses this file
 * to you under the Apache License, Version 2.0 (the
 * "License"); you may not use this file except in compliance
 * with the License.  You may obtain a copy of the License at
 *

httpx.post(callback_url, json={"description": "Invoice paid", "paid": True})
```

But possibly the most important part of the callback is making sure that your API user (the external developer) implements the *external API* correctly, according to the data that *your API* is going to send in the request body of the callback, etc.

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*.

			// client sent this as part of signed headers we need to
			// handle otherwise we would see a signature mismatch.
			// `aws-cli` sets this as part of signed headers.
			//
			// According to
			// http://www.w3.org/Protocols/rfc2616/rfc2616-sec14.html#sec14.20
			// Expect header is always of form:
			//
			//   Expect       =  "Expect" ":" 1#expectation

   * and values are the result of applying the provided mapping functions to the input elements.
   * Entries appear in the result {@code ImmutableMap} in encounter order.
   *
   * <p>If the mapped keys contain duplicates (according to {@link Object#equals(Object)}, an {@code
   * IllegalArgumentException} is thrown when the collection operation is performed. (This differs

   * Entries appear in the result {@code ImmutableBiMap} in encounter order.
   *
   * <p>If the mapped keys or values contain duplicates (according to {@link
   * Object#equals(Object)}), an {@code IllegalArgumentException} is thrown when the collection
   * operation is performed. (This differs from the {@code Collector} returned by {@link

/*
 * Licensed to the Apache Software Foundation (ASF) under one
 * or more contributor license agreements.  See the NOTICE file
 * distributed with this work for additional information
 * regarding copyright ownership.  The ASF licenses this file
 * to you under the Apache License, Version 2.0 (the
 * "License"); you may not use this file except in compliance
 * with the License.  You may obtain a copy of the License at
 *

<?xml version="1.0" encoding="UTF-8"?>

<!--
Licensed to the Apache Software Foundation (ASF) under one
or more contributor license agreements.  See the NOTICE file
distributed with this work for additional information
regarding copyright ownership.  The ASF licenses this file
to you under the Apache License, Version 2.0 (the
"License"); you may not use this file except in compliance
with the License.  You may obtain a copy of the License at