This draft PR implements naming changes described here: googleapis/google-cloud-go#12538 (comment)

In our team API discussion and feedback from other reviewers, there are many changes not applied to Java yet. I will try to list them all here, based on the differences I see in your diagram.

FilterCondition is renamed to BooleanExpr
Accumulator is renamed AggregateFunction
AggregateFunction no longer inherits from Function, but is instead root of separate inheritance tree.
AccumulatorTarget is renamed to AliasedAggregate
AliasedAggregate does not implement Selectable interface
ExprWithAlias is renamed to AliasedExpr
Return values of boolean operators 'Eq', 'Neq', etc should be BooleanExpr (the actual EqCondition >and NeqCondition classes are an implementation detail that should appear in customer code). >Likewise, return values for Add and Subtract should be Expr. And so forth...

We will update this as names continue to evolve

TODO:

• better encapsule expressions and stages from end users?

  • some of them are exposed publicly, but we want them to be more internal/hidden. Maybe move them

• get tests and lint passing

• add tests for new AggregateAlias class