code: private static Set<String> collectIntersectionPrefixes(FieldInfo fi)
        throws IOException {
      if (fi != null) {
        Set<String> result = new HashSet<>();
        String intersectingPrefixes = fi.getAttribute(
            MtasCodecPostingsFormat.MTAS_FIELDINFO_ATTRIBUTE_PREFIX_INTERSECTION);
        if (intersectingPrefixes != null) {
          String[] prefixes = intersectingPrefixes
              .split(Pattern.quote(MtasToken.DELIMITER));
          for (int i = 0; i < prefixes.length; i++) {
            String item = prefixes[i].trim();
            if (!item.equals("")) {
              result.add(item);
            }
          }
        }
        return result;
      } else {
        return Collections.emptySet();
      }
    }
docs: Collect intersection prefixes.
  
  @param fi
  the fi
  @return the sets the
  @throws IOException
  Signals that an I/O exception has occurred.