and the server has commits in lower case that the client
doesn&#8217;t, as in the following diagram:</p>
</div>
<div class="literalblock">
<div class="content">
<pre>      +---- u ---------------------- x
     /              +----- y
    /              /
   a -- b -- c -- d -- E -- F
      \
+--- Q -- R -- S</pre>
</div>
</div>
<div class="paragraph">
<p>If the client wants x,y and starts out by saying have F,S, the server
doesn&#8217;t know what F,S is.  Eventually the client says "have d" and
the server s