Tribool is a Swift micro-library that provides an Obj-C-compatible Tribool enum type, for representing 3-value logic states: .true, .false, and .indeterminate.
The master branch is Swift 5.x, and build overlays (the minimal changeset to the Package.swift, xcodeproj, and other build files) of the current library version are available on the swift-4.2, swift-4, and swift-3 branches. (Note: I don't check that these are built as often as I used to when Swift 4.2 or 4.0 were the latest versions, but their changes haved worked and I've merged new library versions into them since then.)