The unittest fixture for SBE PlanStages should provide a mechanism to create a PlanStage sub-tree representing an input to the PlanStage being tested, and served as a mocked data queue. This can be implemented by means of coscan - limit 1 - project - unwind subtree, where project stage just projects out a constant array holding BSON elements representing input stream of documents, and unwind stage flattens the array into a stream of documents.

Some additional helper methods may be required to simply the execution and result validation of the PlanStage tree.