Cornelis Hermanus Antonius "Kees" Koster (born 13 July 1943[1]) is a professor in the Department of Informatics of the University of Nijmegen in the Netherlands.
He was one of the editors of the original Report on the Algorithmic Language ALGOL 68 and was the creator of the original Compiler Description Language (CDL). He is also the creator of Affix grammars, which are a variant of Van Wijngaarden grammars. In a sense, CDL is a deterministic executable affix grammar, while Prolog is a non-deterministic executable affix grammar (a link acknowledged by the implementors of the original Prolog interpreter).