$transform: {
	root: <expression evaluates to object>,
	set: {<fieldname>: <arbitrary expression>}
}

// =====  Example #1 - Promoting a subfield to the top level =====
>db.example.insert({_id: 0, a1: {b: 1}, a2: 2})
>db.example.aggregate([{$transform: {root: “$a1”}]);
{b: 1}

// =====  Example #2 - Creating a new field =====
>db.example.insert({_id: 0, a1: {b: 1}, a2: 2})
>db.example.aggregate([{$transform: {set: {a3: 3}}}])
{_id: 0, a1: {b: 1}, a2: 2, a3: 3}

// =====  Example #3 - Creating new calculated fields =====
>db.example.insert({_id: 0, a1: {b: 1}, a2: 2})
>db.example.aggregate([{$transform: {set: {a3: {$gt: ["$a2", 3]}, a4: 4 }}}])
{_id: 0, a1: {b: 1}, a2: 2, a3: false, a4: 4}

// =====  Example #4 - Promoting a subfield to the top level and then creating new fields =====
>db.example.insert({_id: 0, a1: {b: 1}, a2: 2 })
>db.example.aggregate([{$transform: {root: "$a1", set: {c: 3}}}])
{b: 1, c: 3}

// =====  Example #5 - Error: using value of a field that doesn't exist anymore =====
>db.example.insert({_id: 0, a1: {b: 1}, a2: 2 })
>db.example.aggregate([{$transform: {root: "$a1", set: {c: "$a2"}}}])
Error
// Allowing this is a non-goal of this ticket.