@Test
public void testAmong() {

  Parser<Void> scanner = Scanners.among("ab");
  assertNull(scanner.parse("a"));
  assertNull(scanner.parse("b"));
  assertScanner(mode, scanner, "ab", "b");
  assertFailure(mode, scanner, "c", 1, 1, "[ab] expected, c encountered.");
  assertFailure(mode, scanner, "", 1, 1, "[ab] expected, EOF encountered.");
}