@Override
    public String toString() {
      if (cachedToString == null) {
        cachedToString = toCharSequence().toString();
      }
      return cachedToString;
    }
Ability to pass captured fragment and evaluate it as a string.
For non-captured fragments or fragments which expects parameters, plain {@code toString}
returned.
generator/src/org/immutables/generator/Templates.java