A $lookup with absorbed $unwind with includeArrayIndex can have multiple output fields.

[{$lookup: {from: 'coll_b', localField: 'x', foreignField: 'y', as: 'e'}},
 {$unwind: {path: '$e', includeArrayIndex: 'idx'}}]

Here the resulting DocumentSourceLookup post-absorbtion outputs "as" and "idx".

The graph does not distinguish between these fields and existing APIs like canPathBeArray assume "idx" contains the results from the subpipeline.

(Thanks to Henri for noticing this!)