A neat trick for refactoring code

So today I'm going to be cleaning up one of the vocabularies involved in code generation in the optimizing compiler. Each vocabulary has a set of words which are used by other vocabularies -- the "public interface", and implementation words, those only used to implement the interface. I want to rewrite the generator.registers vocabulary but keep the public interface as close to its current state as possible. Trouble is, the public interface is not well-defined here; this vocabulary does not use <PRIVATE and PRIVATE>. However, this is not a big problem, since I can just write a tool:

: used-externally ( vocab -- words )
    dup words [
        usage
        [ word? ] subset
        [ word-vocabulary = not ] curry* contains?
    ] curry* subset natural-sort ;

How does this work? It takes the list of words in a vocabulary, then for each word, checks if the usages of that word include any words outside of this vocabulary. If the answer is affirmative, it adds this word to the sequence that is returned.

Using this tool I found out that generator.registers vocabulary only has a handful of words which are used externally, and so it should be easy to do a radical refactoring which doesn't require many changes to other modules.

It is hard to articulate how powerful this is. Not only is the Factor listener is a great way of testing code, it also provides a way to interact with the system on a meta-level. I wonder how much work it would be to implement something like used-externally in a conventional IDE such as Eclipse; I'm sure all the pieces are there (cross-referencing, etc) but it can't be that easy to just glue things together like that.