In the latest version of trgen, a grammar that has two or more top-level grammars cannot work without specifying in the desc.xml to pick one.
In glsl, there is a parser grammar and a parser grammar for a preprocessor. At the moment, the tester cannot pipeline parsers together, i.e., take output of preprocessor and pass to post-preprocessor parser. The desc.xml must specify <grammar-name>GLSL</grammar-name>.