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The Top-Level Loop
The top-level loop normally reads the query from the user, executes
it and prints the answer bindings.
In case that a subgoal remains delayed17.4,
the answer bindings alone are not logically correct,
they have to be considered in conjunction with the remaining delayed goals.
To signal this, the top-level loop first prints the answer bindings
and then it checks whether some goals are still delayed.
If so, an exception is raised; the default handler for this exception
prints the goals that are still delayed, so that the user
is notified about this situation.
This handler can as well print only the number of delayed goals,
or ask the user for some action, e.g. force waking of
some or all delayed goals.
1999-08-06