Complex sentence: It was used mainly as an experimental test system for language design in the 1980s and 1990s; however, as of July of 2006, Self is still being actively developed as part of the Klein project, which is a Self virtual machine written fully in Self.
Split into simpler sentences: It was used mainly as an experimental test system for language design in the 1980s and 1990s. In 2006, Self was still being developed as part of the Klein project, which was a Self virtual machine written fully in Self.