@Test
public void testWord_throwsForNullValue() {

  Parser<?> tokenizer = Terminals.CharLiteral.SINGLE_QUOTE_TOKENIZER;
  Lexicon lexicon = new Lexicon(__ -> null, tokenizer);
  assertSame(tokenizer, lexicon.tokenizer);
  try {
    lexicon.word("whatever");
    fail();
  } catch (IllegalArgumentException e) {}
}