int writeList(List<Span> spans, byte[] out, int pos) {
    int lengthOfSpans = spans.size();
    if (lengthOfSpans == 0) return 0;

    WriteBuffer result = WriteBuffer.wrap(out, pos);
    for (Span span : spans) {
      SPAN.write(result, span);
    }
    return result.pos() - pos;
  }
Encodes per ListOfSpans data wireType, where field one is repeated spans
zipkin/src/main/java/zipkin2/internal/Proto3SpanWriter.java