CLU

CLU was an experimental programming language created to explore abstract data types. Barbara Liskov started to design CLU in 1973, as part of the Computation Structures Group of Project MAC. The computer hosting development was the Dynamic Modeling PDP-10 running ITS. Initial work was done using Maclisp, but it was moved over to the Muddle language, which was integral to the Dynamic Modeling project. Later versions would be increasingly self-hosting.

Bob Scheifler was part of the CLU group for many years prior to the creation of the X Window System. When the group needed a windowing system, he took W and updated it to become X. CLU was one of the first programming languages to get support for X, in fact even before C.