@Test
public void testString() {

  Parser<Void> scanner = Scanners.string("ab");
  assertNull(scanner.parse("ab"));
  assertScanner(mode, scanner, "abc", "c");
  assertFailure(mode, scanner, "c", 1, 1, "ab expected, c encountered.");
  assertFailure(mode, scanner, "a", 1, 1);
  assertFailure(mode, scanner, "", 1, 1);
}