public String getRemainingBuffer() {
			if ( n==0 ) return "";
			int len = n;
			if (data[len-1] == IntStream.EOF) {
				// Don't pass -1 to new String().
				return new String(data,p,len-p-1) + "\uFFFF";
			} else {
				return new String(data,p,len-p);
			}
		}
For testing.  What's in moving window into data stream from
 current index, LA(1) or data[p], to end of buffer?
tool-testsuite/test/org/antlr/v4/test/tool/TestUnbufferedCharStream.java