This all sounds awful close to the whole CLISP/ReadLine debacle. Basically, CLISP (a Lisp implementation) originally linked to ReadLine, a GNU library under the

This all sounds awful close to the whole CLISP/ReadLine debacle. Basically, CLISP (a Lisp implementation) originally linked to ReadLine, a GNU library under the GPL. Richard Stallman argued that the author of CLISP had to remove ReadLine or license CLISP under the GPL. The author originally created his own non-GPL library with the same interface as ReadLine and distributed that, noting that the user could (at their own option) link CLISP with GNU ReadLine instead if they wanted that functionality. Stallman argued that wasn't sufficient. In the end, CLISP ended up being relicensed to GPL. Note though that no judge ever looked at it, so things might have turned out differently if it had gone to court.