private String typaramsString(List<Type> tvars) {
            StringBuilder s = new StringBuilder();
            s.append('<');
            boolean first = true;
            for (Type t : tvars) {
                if (!first) s.append(", ");
                first = false;
                appendTyparamString(((TypeVar)t), s);
            }
            s.append('>');
            return s.toString();
        }