@Override
	public int mark() {
		if (numMarkers == 0) {
			lastTokenBufferStart = lastToken;
		}

		int mark = -numMarkers - 1;
		numMarkers++;
		return mark;
	}
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