@Test
public void testSource() {

  assertEquals("source", FOO.source().toString());
  assertEquals("", FOO.source().parse("", mode));
  assertParser(mode, COMMA.source(), ", ", ",", " ");
  assertParser(mode, Terminals.IntegerLiteral.TOKENIZER.label("INTEGER").source(), "123 ", "123", " ");
  assertEquals("123",
      Parsers.tokenType(Integer.class, "int")
          .from(INTEGER, Scanners.WHITESPACES).source()
          .parse("123", mode));
}