@Test public void test2CharAhead() throws Exception {
   		CharStream input = createStream("xy");
   		assertEquals('x', input.LA(1));
   		assertEquals('y', input.LA(2));
   		assertEquals(IntStream.EOF, input.LA(3));
   	}
It is not valid to pass a mark to {@link IntStream#seek}, but
{@link UnbufferedCharStream} creates marks in such a way that this
invalid usage results in an {@link IllegalArgumentException}.
tool-testsuite/test/org/antlr/v4/test/tool/TestUnbufferedCharStream.java