`brew list --verbose <formulae>` lists only those kegs that
are installed and exits with 0 if all <formulae> installed.
If one or more formulae from <formulae> are not installed, the
command lists only installed versions and exits with 1.
`brew --pinned formula` exits with 1 if formula is not pinned.
Fixes#1172
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.