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$elemMatch children can be simplified as independent Boolean expressions, then the result could be attached to the parent's BitsetTree. We will need to modify `restoreMatchExpression` function to handle that case.
In the $elemMatch's parent's conjunction contains the same terms as the $elemMatch's conjunction we can remove the terms from the $elemtMatch's conjunction. E.g.
```
 { "a": {"id":1}, "a":{$elemMatch:{"id":1, "b":9} } } == { "a": {"id":1}, "a":{$elemMatch:{"b":9} } } 
```
see ~~SERVER-22857~~

related to

SERVER-22857 eliminate redundant conditions/clauses from query

split from

SERVER-75079 Simplify boolean expressions before feeding them to the optimizer

Participants:: [DO NOT USE] Backlog - Query Optimization, Alexander Ignatyev