@Test
public void testSequence_4Parsers() {

  Parser<Void> parser = Parsers.sequence(
      Scanners.isChar('a'), Scanners.isChar('b'), Scanners.isChar('c'), Scanners.isChar('d'));
  assertEquals("sequence", parser.toString());
  assertEquals(null, parser.parse("abcd", mode));
  assertFailure(mode, parser, "xbcd", 1, 1);
  assertFailure(mode, parser, "axcd", 1, 2);
  assertFailure(mode, parser, "abxd", 1, 3);
  assertFailure(mode, parser, "abcx", 1, 4);
}