public List<Grammar> getGrammarAncestors() {
        Grammar root = getOutermostGrammar();
        if ( this==root ) return null;
        List<Grammar> grammars = new ArrayList<Grammar>();
        // walk backwards to root, collecting grammars
        Grammar p = this.parent;
        while ( p!=null ) {
            grammars.add(0, p); // add to head so in order later
            p = p.parent;
        }
        return grammars;
    }
Return list of imported grammars from root down to our parent.
 Order is [root, ..., this.parent].  (us not included).
tool/src/org/antlr/v4/tool/Grammar.java