@Override
    public int mark() {
		return 0;
	}
Indicates whether the {@link Token#EOF} token has been fetched from
{@link #tokenSource} and added to {@link #tokens}. This field improves
performance for the following cases:

<ul>
<li>{@link #consume}: The lookahead check in {@link #consume} to prevent
consuming the EOF symbol is optimized by checking the values of
{@link #fetchedEOF} and {@link #p} instead of calling {@link #LA}.</li>
<li>{@link #fetch}: The check to prevent adding multiple EOF symbols into
{@link #tokens} is trivial with this field.</li>
<ul>
runtime/Java/src/org/antlr/v4/runtime/BufferedTokenStream.java