This is a proper fix to the problem addressed by #1510.
The problem arises when f_kegs is nil, which can happen if the name and
tap used to install a keg don't match the name and tap currently
associated with its formula (i.e. if it's been renamed or moved).
This is the (partially) documented public API for Homebrew.
The main class you should look at is the {Formula} class (and classes linked from there). That's the class that's used to create Homebrew formulae (i.e. package descriptions). Assume anything else you stumble upon is private.