This task is listed as "optional" in the "Lower physical tree to SBE plan" scope document.

’find({a: 5}, {'x.$': 1})’ records the index k of the first element of array “a” that is equal to 5, and then accesses the kth element of array “x”.

This feature is outside of the scope of CQF Milestone 2, but this is an important thing to explore in order to prove that our model for the SBE Plan Generator will continue to work well for future CQF milestones.