code: @Test public void writePrefixDoesntSplitButRequiresCompact() throws Exception {
    OkBuffer sink=new OkBuffer();
    sink.writeUtf8(repeat('b',Segment.SIZE - 10));
    sink.readUtf8(Segment.SIZE - 20);
    OkBuffer source=new OkBuffer();
    source.writeUtf8(repeat('a',Segment.SIZE * 2));
    sink.write(source,20);
    assertEquals(asList(30),sink.segmentSizes());
    assertEquals(asList(Segment.SIZE - 20,Segment.SIZE),source.segmentSizes());
    assertEquals(30,sink.size());
    assertEquals(Segment.SIZE * 2 - 20,source.size());
  }
  