Rudder

It should be easy to create new generic methods by compositing existing methods.
To ease this, we should provide methods to easily aggregate other method outcome into a new one.

Here are the rules (+ if one has not been reached, just copy the other one to the destination).

repaired = repaired+repaired ou repaired+kept = repaired + ok
kept = kept+kept
ok = ok+ok
reached = reached + *
failed = failed + *
denied = denied + *
timeout = timeout + *
error = error + *
not_ok = !ok
not_kept = !kept
not_repaired = !repaired