The `build` and `source_modified_time` arguments are always coming from
the matching attributes of the `Formula` instance. Thus query `formula`
for them instead of passing them individually.
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.