is procedure.
</p>
<p>If the optional &rsquo;handler&rsquo; argument is provided, then that handler will
run if &rsquo;thunk&rsquo; throws, and the return value of the handler would
become the return value of this procedure; otherwise the program will
terminate if an unhandled exception propagates out of &rsquo;thunk&rsquo;.
&rsquo;handler&rsquo; should take the same arguments as a guile catch handler
(this is implemented using catch).  Note that unlike a handler passed
to the thread-pool-add! procedure, &r