@Override public List<List<Span>> map(List<List<Span>> input) {
    int length = input.size();
    if (length == 0) return input;
    ArrayList<List<Span>> result = new ArrayList<>(length);
    for (List<Span> next : input) {
      if (request.test(next)) result.add(next);
    }
    return result;
  }
Filters the mutable input based on the query
zipkin/src/main/java/zipkin2/internal/FilterTraces.java