@Test
	public void testSeekPastEOF() {
		CharStream input = createStream("");
		assertEquals(0, input.index());
		input.seek(1);
		assertEquals(0, input.index());
	}
The {@link IntStream} interface does not specify the behavior when the
EOF symbol is consumed, but {@link UnbufferedCharStream} handles this
particular case by throwing an {@link IllegalStateException}.
tool-testsuite/test/org/antlr/v4/test/tool/TestUnbufferedCharStream.java