'Add fact k=v to the knowledge base.
Returns True if the KB has actually been updated, False otherwise.'

def _tell(self, k, v):

    if ((k in self) and (self[k] is not None)):
        if (self[k] == v):
            return False
        else:
            raise InconsistentAssumptions(self, k, v)
    else:
        self[k] = v
        return True
