ocking calls
should not be made in &rsquo;thunk&rsquo;.
</p>
<p>This procedure can be used for the purpose of implementing
co-operative multi-tasking.  However, when &rsquo;thunk&rsquo; is executed, this
procedure is waiting on &rsquo;await&rsquo;, so &rsquo;await&rsquo; and &rsquo;resume&rsquo; cannot be
used again in &rsquo;thunk&rsquo; (although &rsquo;thunk&rsquo; can call a-sync to start
another series of asynchronous operations with a new await-resume
pair).  For that reason, await-yield! is usually m