title: racket cover gets overwritten if I execute more than one set of tests
body: <p>I am trying to measure coverage with racket cover</p>
  <p><a href="https://docs.racket-lang.org/cover/basics.html" rel="nofollow noreferrer">https://docs.racket-lang.org/cover/basics.html</a></p>
  <p>like this</p>
  <pre><code>raco cover hello.rkt
  </code></pre>
  <p>However, if I run it more than once, it overwrites what is in the coverage directory instead of adding to it.  If I do something like</p>
  <pre><code>raco cover hello.rkt
  raco cover bye.rkt
  </code></pre>
  <p>the two html reports specific to these rkt files (coverage/hello.html and coverage/bye.html) remain intact, coverage/index.html will only have bye.rkt in it.  Also I guess if they share a library, I'll only get coverage info for that library from the last run.</p>
  <p>Does anybody have a way to combine two coverage reports from two different executions?</p>