code: function replaceOne(coll, filter, doc, options, callback) {
    // Set single document update
    options.multi = false;
  
    // Execute update
    updateDocuments(coll, filter, doc, options, (err, r) => {
      if (callback == null) return;
      if (err && callback) return callback(err);
      if (r == null) return callback(null, { result: { ok: 1 } });
  
      r.modifiedCount = r.result.nModified != null ? r.result.nModified : r.result.n;
      r.upsertedId =
        Array.isArray(r.result.upserted) && r.result.upserted.length > 0
          ? r.result.upserted[0] // FIXME(major): should be `r.result.upserted[0]._id`
          : null;
      r.upsertedCount =
        Array.isArray(r.result.upserted) && r.result.upserted.length ? r.result.upserted.length : 0;
      r.matchedCount =
        Array.isArray(r.result.upserted) && r.result.upserted.length > 0 ? 0 : r.result.n;
      r.ops = [doc];
      if (callback) callback(null, r);
    });
  }
docs: Replace a document in the collection.
  
  @method
  @param {Collection} a Collection instance.
  @param {object} filter The Filter used to select the document to update
  @param {object} doc The Document that replaces the matching document
  @param {object} [options] Optional settings. See Collection.prototype.replaceOne for a list of options.
  @param {Collection~updateWriteOpCallback} [callback] The command result callback