@Test
public void testFrom_throwsOnScanners() {

  assertFailure(mode, string("foo").from(constant(Arrays.asList(new Token(0, 3, "foo")))),
      "foo", 1, 1, "Cannot scan characters on tokens.");
  assertFailure(mode, isChar('f').from(constant(Arrays.asList(new Token(0, 1, 'f')))),
      "f", 1, 1, "Cannot scan characters on tokens.");
}