For example, say you want all documents with key A in collA such that there are no documents in collB with key B = A. Expressing that in MQL can be done (inefficiently) as:

{$lookup: {
  from: collB,
  localField: A,
  foreignField: B,
  as: b
}},
{$match: {b: {$eq: []}}}

Maybe we could have like a $inverseLookup or a $diff or similar (even as an internal stage) to avoid materializing the results of a $lookup just so we can discard them in the next stage.