> Do you know about opensource.apple.com, GNUstep, The Cocotron and other projects?

> We do, and in fact, Darling is largely based on the original Darwin source code published by Apple. We use The Cocotron as a basis for our Cocoa implementation, along with the Apportable Foundation and various bits of GNUstep.

Apportable (<3) was a YC startup https://www.ycombinator.com/companies/apportable from 2011 - the Foundation repo last update was Dec 2014.