Written in Modula-2. I had to learn Modula-2 in university. When I read the announcement that we were to use Modula I thought it was Modula-3 that was already a thing at the time. But I guess Modula-2 is good enough and an even better teaching language. It was also a bit popular on the Atari ST with the Megamax Modula compiler (and at least three other Modula compilers!)

EDIT: not written in Modula-2 but wrongly attributed by Github.