Called by ABSTRACT_BACKTRACKING to let
the current object switch to the next alternative
to explore.
Two actions must be performed by that feature:
- make iteration actions and then call
'continue_alternative' if iteration is not ended
and that current object must be called again to
switch to an other alternative or else call
'pop_alternative' if this is the last alternative
to be explored.
- select the state to be evaluated or
call 'continue' or call 'backtrack'.