code: protected ClusKernel emptyBuffer(long currentTime, double negLambda) {
          this.buffer.makeOlder(currentTime - this.timestamp, negLambda);
          ClusKernel bufferCopy = new ClusKernel(this.buffer);
          this.buffer.clear();
          return bufferCopy;
      }
docs: Clear the buffer in this entry and return a copy. No side effects are
  possible (given that the copy constructor of <code>Kernel</code> makes
  a deep copy).
  @return A copy of the buffer.