If we have a predicate like (ComposeM PathArr (Traverse PathArr)), we should simplify it to just (Traverse PathArr). Traverse means "is or contains", so if a value "is-or-contains an array" then it must be an array. We can remove PathArr because it's implied by (Traverse PathArr).

This will allow more predicates to be covered by an index.

For example, here is a query that would benefit:

db.c.find({a: {$elemMatch: {$elemMatch: {$eq: 2}}}})

In other words: 'a' is an array, which contains an array, which contains 2.

It becomes this ABT:

Get a
ComposeM
|   PathArr
Traverse
ComposeM
|   PathArr
Traverse
PathCompare Eq 2

After relaxing the predicates we get this Sargable node:

Get a Identity -> [ [], BinData )   # 'a' is an array
Get a Traverse Identity -> [ [], BinData )  # 'a' is-or-contains an array
Get a Traverse Traverse Identity -> [ 2, 2 ]  # 'a' is-or-contains something which is-or-contains 2

A multikey index on {a: 1} can cover the second and third predicates, but not the first. But the second predicate implies the first, so we should just drop the first predicate, or mark it perfOnly.

Index path: Get "a" Traverse id.