When setting or getting fields in our expressions API, the resulting MQL is verbose, since it produces $getField and $setField MQL. 

• It should be possible to use a post-processor to collapse some of these into simple string paths. 
• For literal documents, it should be possible to execute the gets and sets in the "local execution context" (storing and producing a new internal document each time, rather than generating composite expressions that wrap the original literal document).