BSONArrayBuilder execShardsBuilder(executionStagesBob.subarrayStart("shards"));
    for (size_t i = 0; i < shardResponses.size(); i++) {
        auto responseData = shardResponses[i].swResponse.getValue().data;

        BSONObjBuilder singleShardBob(execShardsBuilder.subobjStart());
        BSONObj execStats = responseData["executionStats"].Obj();

        singleShardBob.append("shardName", shardResponses[i].shardId.toString());
        appendElementsIfRoom(&singleShardBob, execStats);
        singleShardBob.doneFast();
    }
    execShardsBuilder.doneFast();

    executionStagesBob.doneFast();

    if (!firstShardResponseData["executionStats"].Obj().hasField("allPlansExecution")) {
        // The shards don't have execution stats for all plans, so we're done.
        executionStatsBob.doneFast();
        return;
    }

    // Add the allPlans stats from each shard.
    BSONArrayBuilder allPlansExecBob(executionStatsBob.subarrayStart("allPlansExecution"));    // NOTE(ianb): Notice how 'allPlansExecBob' is never used.
    for (size_t i = 0; i < shardResponses.size(); i++) {
        auto responseData = shardResponses[i].swResponse.getValue().data;
        // NOTE(ianb): Notice how we're using 'execShardsBuilder' even though we already called done() on that above.
        BSONObjBuilder singleShardBob(execShardsBuilder.subobjStart());
        singleShardBob.append("shardName", shardResponses[i].shardId.toString());

        BSONObj execStats = responseData["executionStats"].Obj();
        vector<BSONElement> allPlans = execStats["allPlansExecution"].Array();

        BSONArrayBuilder innerArrayBob(singleShardBob.subarrayStart("allPlans"));
        for (size_t j = 0; j < allPlans.size(); j++) {
            appendToArrayIfRoom(&innerArrayBob, allPlans[j]);
        }
        innerArrayBob.done();

        singleShardBob.doneFast();
    }
    allPlansExecBob.doneFast();

    executionStatsBob.doneFast();
}