Recent commit 36dbad3922 to default to frozen string literals (via `frozen_string_literal`) broke `check_for_unsupported_macos()` on outdated releases--due to attempted modification of a frozen string literal.
This breaks `install` and `doctor`.
It's like the change wasn't tested on an unsupported outdated release or something... :D
Adding a `+` prefix is apparently one way around this issue according to https://stackoverflow.com/questions/37799296/what-does-the-comment-frozen-string-literal-true-do#37799399. The change worked for me.
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.