code: public Map<String, MissingRevisions> revsDiff(Map<String, Set<String>> revisions) {
          Misc.checkNotNull(revisions, "Input revisions");
          URI uri = this.uriHelper.revsDiffUri();
          String payload = JSONUtils.toJson(revisions);
  
          HttpConnection connection = Http.POST(uri, "application/json");
          connection.setRequestBody(payload);
          return executeToJsonObjectWithRetry(connection, JSONUtils.STRING_MISSING_REVS_MAP_TYPE_DEF);
      }
docs: Returns the subset of given the documentId/revisions that are not stored in the database.
  
  The input revisions is a map, whose key is document ID, and value is a list of revisions.
  An example input could be (in JSON format):
  
  { "03ee06461a12f3c288bb865b22000170":
  [
  "1-b2e54331db828310f3c772d6e042ac9c",
  "2-3a24009a9525bde9e4bfa8a99046b00d"
  ],
  "82e04f650661c9bdb88c57e044000a4b":
  [
  "3-bb39f8c740c6ffb8614c7031b46ac162"
  ]
  }
  
  The output is in same format.
  
  If the ID has no missing revision, it should not appear in the Map's key set. If all IDs
  do not have missing revisions, the returned Map should be empty map, but never null.
  
  @see
  <a target="_blank" href="http://wiki.apache.org/couchdb/HttpPostRevsDiff">HttpPostRevsDiff
  documentation</a>