Only once in my life have I encountered a programming language that was too simple to use. That was Lispkit Lisp, developed by Peter Henderson, Geraint Jones, and Simon Jones, which I saw while serving as a postdoc at Oxford, 1983–87, and which despite its simplicity was used to implement an entire operating system. It is an indightment of the field of programming languages that I have not since encountered another system that I consider too simple . Until today. I can now add a second system to the list of those that are too simple , the appropriately-titled Simplicity, developed by Russell O'Connor of Blockstream. It is described by a paper here and a website here .