@Override
    public List<List<Span>> map(List<List<Span>> input) {
      Iterator<List<Span>> i = input.iterator();
      while (i.hasNext()) { // Not using removeIf as that's java 8+
        List<Span> next = i.next();
        if (!traceIds.contains(next.get(0).traceId())) {
          i.remove();
        }
      }
      return input;
    }
Returns a function that filters its mutable input when it contains a trace not matching the
specified trace IDs.

<p>Make sure the input IDs are unique and {@link Span#normalizeTraceId(String) normalized}.
zipkin/src/main/java/zipkin2/storage/StrictTraceId.java