@Override
    public String apply(Object input) {
      String inputString = String.valueOf(input);
      if (inputString.isEmpty()) {
        return "_";
      }
      String safeString = nonIdentifierChars.replaceFrom(inputString, '_');
      if (digits.matches(safeString.charAt(0))) {
        return "_" + safeString;
      }
      return safeString;
    }
Built-in operations, functions and accessors.
Planned to be moved to import/implicit resolution
rather that inheritance.
generator/src/org/immutables/generator/Builtins.java