gistfile1.txt

 * The key of the array might to be changed. In this case, the new id property needs to be the key.
 * It'd be possible to just add key: "@id" to the iterator process but that
 * would require special casing the key property and somehow escape it when
 * there is actually a destination called key. Instead, it is moved out of the
 * process and key is in the configuration of iterator itself:
 * @code
 * process:
 *   filters:
 *     plugin: iterator
 *     source: filters
 *     key: "@id"
 *     process:
 *       id:
 *         plugin: static_map
 * @endcode
 *
 * The process pipeline for key is special in two ways: first, it runs after
 * the normal process is done. Because of this, key: "@key" is also valid.
 * Second, the pipeline does not start with NULL but the value of the current key.
 * This way, the value of key can come from the other properties or the current
 * key. For the latter, omit the source from the first plugin in the pipeline
 * for key.