private Collection<String> findUsages(String grammarFileName) {
        Collection<String> result = new ArrayList<String>();
        explore(grammarFileName, result);

        return result;
    }
Returns the grammar file names that directly or indirectly use the given grammar.

@param   grammarFileName  the grammar file name.

@return  the grammar file names that use the given grammar file.
antlr4-maven-plugin/src/main/java/org/antlr/mojo/antlr4/GrammarDependencies.java