'Add a list of background assumptions for reasoning about the discourse.
When called,  this method also updates the discourse model\'s set of readings and threads.
:param background: Formulas which contain background information
:type background: list(Expression)'

def add_background(self, background, verbose=False):

    from nltk.sem.logic import Expression
    for (count, e) in enumerate(background):
        assert isinstance(e, Expression)
        if verbose:
            print(('Adding    assumption    %s    to    background' % count))
        self._background.append(e)
    self._construct_readings()
    self._construct_threads()
