@Override
	public int size() {
		throw new UnsupportedOperationException("Unbuffered stream cannot know its size");
	}
Return a marker that we can release later.

<p>The specific marker value used for this class allows for some level of
protection against misuse where {@code seek()} is called on a mark or
{@code release()} is called in the wrong order.</p>
runtime/Java/src/org/antlr/v4/runtime/UnbufferedTokenStream.java