private void explore(String grammarName, Collection<String> result) {
        for (Graph.Node<String> node : graph.getNode(grammarName).edges) {
            result.add(node.payload);
            explore(node.payload, 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