Routes writes into a real stack env's frame chain, the same way <<- reaches an outer scope. Each write also gets a Reads edge back to rootId, so slicing does not drop the call that made it.
<<-
Reads
rootId
Optional
Routes writes into a real stack env's frame chain, the same way
<<-reaches an outer scope. Each write also gets aReadsedge back torootId, so slicing does not drop the call that made it.