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/// @startDocuBlock REST_DOCUMENT_UPDATE
/// @brief updates a document
///
/// @RESTHEADER{PATCH /_api/document/{document-handle},Update document}
///
/// @RESTALLBODYPARAM{document,json,required}
/// A JSON representation of a document update as an object.
///
/// @RESTURLPARAMETERS
///
/// @RESTURLPARAM{document-handle,string,required}
/// This URL parameter must be a document handle.
///
/// @RESTQUERYPARAMETERS
///
/// @RESTQUERYPARAM{keepNull,boolean,optional}
/// If the intention is to delete existing attributes with the patch
/// command, the URL query parameter *keepNull* can be used with a value
/// of *false*. This will modify the behavior of the patch command to
/// remove any attributes from the existing document that are contained
/// in the patch document with an attribute value of *null*.
///
/// @RESTQUERYPARAM{mergeObjects,boolean,optional}
/// Controls whether objects (not arrays) will be merged if present in
/// both the existing and the patch document. If set to *false*, the
/// value in the patch document will overwrite the existing document's
/// value. If set to *true*, objects will be merged. The default is
/// *true*.
///
/// @RESTQUERYPARAM{waitForSync,boolean,optional}
/// Wait until document has been synced to disk.
///
/// @RESTQUERYPARAM{ignoreRevs,boolean,optional}
/// By default, or if this is set to *true*, the *_rev* attributes in 
/// the given document is ignored. If this is set to *false*, then
/// the *_rev* attribute given in the body document is taken as a
/// precondition. The document is only updated if the current revision
/// is the one specified.
///
/// @RESTQUERYPARAM{returnOld,boolean,optional}
/// Return additionally the complete previous revision of the changed 
/// document under the attribute *old* in the result.
///
/// @RESTQUERYPARAM{returnNew,boolean,optional}
/// Return additionally the complete new document under the attribute *new*
/// in the result.
///
/// @RESTHEADERPARAMETERS
///
/// @RESTHEADERPARAM{If-Match,string,optional}
/// You can conditionally update a document based on a target revision id by
/// using the *if-match* HTTP header.
///
/// @RESTDESCRIPTION
/// Partially updates the document identified by *document-handle*.
/// The body of the request must contain a JSON document with the
/// attributes to patch (the patch document). All attributes from the
/// patch document will be added to the existing document if they do not
/// yet exist, and overwritten in the existing document if they do exist
/// there.
///
/// Setting an attribute value to *null* in the patch document will cause a
/// value of *null* to be saved for the attribute by default.
///
/// If the *If-Match* header is specified and the revision of the
/// document in the database is unequal to the given revision, the
/// precondition is violated.
///
/// If *If-Match* is not given and *ignoreRevs* is *false* and there
/// is a *_rev* attribute in the body and its value does not match
/// the revision of the document in the database, the precondition is
/// violated.
///
/// If a precondition is violated, an *HTTP 412* is returned.
///
/// If the document exists and can be updated, then an *HTTP 201* or
/// an *HTTP 202* is returned (depending on *waitForSync*, see below),
/// the *ETag* header field contains the new revision of the document
/// and the *Location* header contains a complete URL under which the
/// document can be queried.
///
/// Optionally, the query parameter *waitForSync* can be used to force
/// synchronization of the updated document operation to disk even in case
/// that the *waitForSync* flag had been disabled for the entire collection.
/// Thus, the *waitForSync* query parameter can be used to force synchronization
/// of just specific operations. To use this, set the *waitForSync* parameter
/// to *true*. If the *waitForSync* parameter is not specified or set to
/// *false*, then the collection's default *waitForSync* behavior is
/// applied. The *waitForSync* query parameter cannot be used to disable
/// synchronization for collections that have a default *waitForSync* value
/// of *true*.
///
/// The body of the response contains a JSON object with the information
/// about the handle and the revision. The attribute *_id* contains the
/// known *document-handle* of the updated document, *_key* contains the
/// key which uniquely identifies a document in a given collection, and
/// the attribute *_rev* contains the new document revision.
///
/// If the query parameter *returnOld* is *true*, then
/// the complete previous revision of the document
/// is returned under the *old* attribute in the result.
///
/// If the query parameter *returnNew* is *true*, then
/// the complete new document is returned under
/// the *new* attribute in the result.
///
/// If the document does not exist, then a *HTTP 404* is returned and the
/// body of the response contains an error document.
///
/// @RESTRETURNCODES
///
/// @RESTRETURNCODE{201}
/// is returned if the document was updated successfully and
/// *waitForSync* was *true*.
///
/// @RESTRETURNCODE{202}
/// is returned if the document was updated successfully and
/// *waitForSync* was *false*.
///
/// @RESTRETURNCODE{400}
/// is returned if the body does not contain a valid JSON representation
/// of a document. The response body contains
/// an error document in this case.
///
/// @RESTRETURNCODE{404}
/// is returned if the collection or the document was not found.
///
/// @RESTRETURNCODE{412}
/// is returned if the precondition was violated. The response will
/// also contain the found documents' current revisions in the *_rev*
/// attributes. Additionally, the attributes *_id* and *_key* will be
/// returned.
///
/// @EXAMPLES
///
/// Patches an existing document with new content.
///
/// @EXAMPLE_ARANGOSH_RUN{RestDocumentHandlerPatchDocument}
///     var cn = "products";
///     db._drop(cn);
///     db._create(cn);
///
///     var document = db.products.save({"one":"world"});
///     var url = "/_api/document/" + document._id;
///
///     var response = logCurlRequest("PATCH", url, { "hello": "world" });
///
///     assert(response.code === 202);
///
///     logJsonResponse(response);
///     var response2 = logCurlRequest("PATCH", url, { "numbers": { "one": 1, "two": 2, "three": 3, "empty": null } });
///     assert(response2.code === 202);
///     logJsonResponse(response2);
///     var response3 = logCurlRequest("GET", url);
///     assert(response3.code === 200);
///     logJsonResponse(response3);
///     var response4 = logCurlRequest("PATCH", url + "?keepNull=false", { "hello": null, "numbers": { "four": 4 } });
///     assert(response4.code === 202);
///     logJsonResponse(response4);
///     var response5 = logCurlRequest("GET", url);
///     assert(response5.code === 200);
///     logJsonResponse(response5);
///   ~ db._drop(cn);
/// @END_EXAMPLE_ARANGOSH_RUN
///
/// Merging attributes of an object using `mergeObjects`:
///
/// @EXAMPLE_ARANGOSH_RUN{RestDocumentHandlerPatchDocumentMerge}
///     var cn = "products";
///     db._drop(cn);
///     db._create(cn);
///
///     var document = db.products.save({"inhabitants":{"china":1366980000,"india":1263590000,"usa":319220000}});
///     var url = "/_api/document/" + document._id;
///
///     var response = logCurlRequest("GET", url);
///     assert(response.code === 200);
///     logJsonResponse(response);
///
///     var response = logCurlRequest("PATCH", url + "?mergeObjects=true", { "inhabitants": {"indonesia":252164800,"brazil":203553000 }});
///     assert(response.code === 202);
///
///     var response2 = logCurlRequest("GET", url);
///     assert(response2.code === 200);
///     logJsonResponse(response2);
///
///     var response3 = logCurlRequest("PATCH", url + "?mergeObjects=false", { "inhabitants": { "pakistan":188346000 }});
///     assert(response3.code === 202);
///     logJsonResponse(response3);
///
///     var response4 = logCurlRequest("GET", url);
///     assert(response4.code === 200);
///     logJsonResponse(response4);
///   ~ db._drop(cn);
/// @END_EXAMPLE_ARANGOSH_RUN
/// @endDocuBlock
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