The ``vol_i`` parameter refers to the *exploded* volume, the one beeing
connected to the ambient. Two kinds of parameters are allowed:

* and integer refering to an existing volume ID
* a string "<id1> + <id2> [ + <idn>...] refering to existing volume IDs

When volume IDs are *summed*, ESonix will automatically merge those volumes
and reroute connections.

.. index:: regulation, FAR25.365, opening hole