Common interface for all internal actions.
Returns true if the internal action (IA) should suspend the
intention where the IA is called
Return true if the internal action can be used in plans' context
Prepare body's terms to be used in 'execute', normally it consist of cloning and applying each term
Executes the internal action. It should return a Boolean or
an Iterator<Unifier>. A true boolean return means that the IA was
successfully executed. An Iterator result means that there is
more than one answer for this IA (e.g. see member internal action).