Choose N tasks out from M paths in fork nodes

For example:

<fork name="fork1">
<transition name="to A" to="A">
</transition>
<transition name="to B" to="B">
</transition>
<transition name="to C" to="C">
</transition>
<transition name="to D" to="D">
</transition>
</fork>

This issue means that when process to this fork, only some of this 4 transition can be chosen to go,
for example ,the transition "to A" and "to A".