var e = a.find().explain("executionStats").executionStats;
assert.eq(3, e.nReturned, "ex1");
assert.eq(0, e.totalKeysExamined, "ex2");
assert.eq(4, e.totalDocsExamined, "ex3");

var chunkSkips = 0;
for (var shard in e.executionStages.shards) {
    var theShard = e.executionStages.shards[shard];
    chunkSkips += getChunkSkips(theShard.executionStages);
}
assert.eq(1, chunkSkips, "ex4"); // <----- fails

[js_test:shard3] uncaught exception: Error: [1] != [0] are not equal : ex4 :
[js_test:shard3] doassert@src/mongo/shell/assert.js:20:14
[js_test:shard3] assert.eq@src/mongo/shell/assert.js:179:9
[js_test:shard3] @jstests/sharding/shard3.js:91:1
[js_test:shard3] @jstests/sharding/shard3.js:1:2
[js_test:shard3] failed to load: jstests/sharding/shard3.js
[js_test:shard3] exiting with code -3

/**
 * Get the number of chunk skips for the BSON exec stats tree rooted at 'root'.
 */
function getChunkSkips(root) {
    if (root.stage === "SHARDING_FILTER") {
        return root.chunkSkips;
    } else if ("inputStage" in root) {
        return getChunkSkips(root.inputStage);
    } else if ("inputStages" in root) {
        var skips = 0;
        for (var i = 0; i < root.inputStages.length; i++) {
            skips += getChunkSkips(root.inputStages[0]);
        }
        return skips;
    }

    return 0;
}