The current PBT grammars are in this folder. This ticket will require adding support for more agg expressions in these grammars, as there’s currently extremely limited support for them (possibly only FieldPathExpressions). It will be hard/counter-productive to write a comprehensive grammar for all agg expressions, so this ticket should implement some reasonable subset of expressions which are meaningful under $expr. We will first use this to increase coverage of match-splitting, so we should at least include logical and comparison operators. Then, we should extend the $match grammar to support generating $expr expressions using these agg expressions. 

As part of this ticket, we’ll need to experiment to see whether these new expressions are compatible with pre-existing PBTs, or if we need to provide a way to selectively disable $match with $expr.