protected List<? extends Token> getBuffer() {
			if ( n==0 ) {
				return Collections.emptyList();
			}

			return Arrays.asList(tokens).subList(0, n);
		}
For testing.  What's in moving window buffer into data stream.
 From 0..p-1 have been consume.
tool-testsuite/test/org/antlr/v4/test/tool/TestUnbufferedTokenStream.java