Of course it's fine to save warn() for "real" error messages, but then the same could be said for only using print() for real output messages.
My problem, for which I'd welcome suggestions, is that I'm not familiar with how to use debug() in the midst of debugging a huge Tk-based application that I'm working on. The subroutines I am debugging get fired when I click one of the Tk buttons, and I know for sure that I can get at my warn() messages as above via my STDERR files. Out of ignorance, I'm just expecting use of debug() to be impractical.
