B$.7[20]$ B$\mathrm{.}1,$ B$.3>$ In systems with a write$-$through L$1$ cache backed by a write$-$back L$2$ cache instead of main memory, a merging write buffer can be simplified. Explain how this can be done. Are there situations where having a full write buffer $($instead of the simple version you have just proposed$)$ could be helpful?