[gnulinux]

Because it says:

> Plankalkül was more comprehensively published [vague] in 1972. The first compiler was implemented by Joachim Hohmann in his 1975 dissertation. Other independent implementations followed in 1998 and 2000 at the Free University of Berlin.

So it was never implemented until mid-1970s, which likely made its design process less constrained than languages that came soon after it in 1950s etc.