c.save( { a:[ 1, 2, 3 ], z:[ 11, 12, 13 ] } );

// The first two elements of 'a' are returned.
printjson( c.find( { z:13 }, { a:{ $slice:2 } } ).toArray() );

// The third element of 'a' is returned (incorrectly), because the third element of 'z' is matched.
printjson( c.find( { z:13 }, { a:{ $slice:2 }, 'z.$':1 } ).toArray() );

> c.save( { a:[ { b:1, c:2 }, { b:3, c:4 } ], z:[ 11, 12, 13 ] } )

// Expected projection for a.b:1.
> c.find( { z:12 }, { 'a.b':1 } )
{ "_id" : ObjectId("503aeb6760273eae2b9d57f7"), "a" : [ { "b" : 1 }, { "b" : 3 } ] }

// In this case, the positional match is applied to the 'a' array as well as the 'z' array.
> c.find( { z:12 }, { 'a.b':1, 'z.$':1 } )
{ "_id" : ObjectId("503aeb6760273eae2b9d57f7"), "a" : [ { "b" : 3, "c" : 4 } ], "z" : [ 12 ] }

// In this case, the query matched 'z' index 2, which does not exist in the 'a' array, and an error is triggered.
> c.find( { z:13 }, { 'a.b':1, 'z.$':1 } )
error: { "$err" : "positional operator element mismatch", "code" : 16353 }

> c.save( { a:[ { b:1, c:2 }, { b:3, c:4 } ] } )
> c.find( { 'a.b':3 }, { 'a.b':1 } )
{ "_id" : ObjectId("503af2d560273eae2b9d57f9"), "a" : [ { "b" : 1 }, { "b" : 3 } ] }

// In this case the a.$ currently supersedes the a.b.  But we might want to uassert instead.
> c.find( { 'a.b':3 }, { 'a.b':1, 'a.$':1 } )
{ "_id" : ObjectId("503af2d560273eae2b9d57f9"), "a" : [ { "b" : 3, "c" : 4 } ] }