@Test
public void testBlockComment() {

  Parser<Void> scanner = Scanners.blockComment("<<", ">>");
  assertNull(scanner.parse("<< this is a comment >>"));
  assertNull(scanner.parse("<<< another comment >>"));
  assertNull(scanner.parse("<<< \"comment\" again >>"));
  assertScanner(mode, scanner, "<<comment>>not comment>>", "not comment>>");
  assertFailure(mode, scanner, "", 1, 1);
  assertFailure(mode, scanner, "<<a >", 1, 6);
}