Narrowing tests for a PathMatchExpression list their documents by hand. That makes it easy to miss traversal cases: arrays and nested arrays at each path component, missing components, and every BSON type at each component.

We can add:
 - Type::hasValue(Value) - returns whether a value is one of the values the type describes

 - A utility that generates documents that put a representative value of every BSON type at each component of the path. 

 - A utility that performs a soundness check - ensures that the inferred type for a $match contains the values that matched.