Thanks for clearing up my last question. I finished coding up the changes & tests for the un-definition case today.

The specification touches on the parallels between definitions and aliases, but doesn't mention a prohibition against using alias to bind an id that is already bound in the current body, or imported by the current top-level library/program body. Do you think it's necessary to state that, or should it be obvious?

The spec says "The alias definition is used to transfer the binding of one identifier to another". The word transfer really implies that something is moved from one place to another, and is therefore no longer in the original place. Maybe copy would be a better term?

splicing-let-alias ? Just a thought. I have it in my system, and it has some use cases, but none that couldn't be effectively replaced with splicing-let-syntax.