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        In case user knows of it.

    - or FACT_SIGN_NEGATIVE
        In case user doesn't knows of it.

    ~ or FACT_SIGN_UNSURE
        In case user doesn't have any clue about the given fact.

  get_rule_by_goal($goal)
    Looks in the knowledge_db for the rule that has the given goal. If a
    rule is found its number is returned, otherwise undef.

  forward()
        use AI::ExpertSystem::Advanced;
        use AI::ExpertSystem::Advanced::KnowledgeDB::Factory;

        my $yaml_kdb = AI::ExpertSystem::Advanced::KnowledgeDB::Factory->new('yaml',
                {
                    filename => 'examples/knowledge_db_one.yaml'
                });

        my $ai = AI::ExpertSystem::Advanced->new(
                viewer_class => 'terminal',
                knowledge_db => $yaml_kdb,
                initial_facts => ['F', 'J']);
        $ai->forward();
        $ai->summary();

    The forward chaining algorithm is one of the main methods used in Expert
    Systems. It starts with a set of variables (known as initial facts) and
    reads the available rules.

    It will be reading rule by rule and for each one it will compare its
    causes with the initial, inference and asked facts. If all of these
    causes are in the facts then the rule will be shoot and all of its goals
    will be copied/converted to inference facts and will restart reading
    from the first rule.

  backward()
        use AI::ExpertSystem::Advanced;
        use AI::ExpertSystem::Advanced::KnowledgeDB::Factory;

        my $yaml_kdb = AI::ExpertSystem::Advanced::KnowledgeDB::Factory->new('yaml',
            {
                filename => 'examples/knowledge_db_one.yaml'
                });

        my $ai = AI::ExpertSystem::Advanced->new(
                viewer_class => 'terminal',
                knowledge_db => $yaml_kdb,
                goals_to_check => ['J']);
        $ai->backward();
        $ai->summary();

    The backward algorithm starts with a set of *assumed* goals (facts). It
    will start reading goal by goal. For each goal it will check if it
    exists in the initial, inference and asked facts (see
    is_goal_in_our_facts()) for more information).

    *   If the goal exist then it will be removed from the dictionary, it
        will also verify if there are more visited rules to shoot.

        If there are still more visited rules to shoot then it will check
        from what rule the goal comes from, if it was copied from a rule
        then this data will exist. With this information then it will see
        how many of the causes of this given rule are still in the
        goals_to_check_dict.

        In case there are still causes of this rule in goals_to_check_dict
        then the amount of causes pending will be reduced by one. Otherwise
        (if the amount is 0) then the rule of this last removed goal will be
        shoot.

    *   If the goal doesn't exist in the mentioned facts then the goal will
        be searched in the goals of every rule.

        In case it finds the rule that has the goal, this rule will be
        marked (added) to the list of visited rules (visited_rules) and also
        all of its causes will be added to the top of the
        goals_to_check_dict and it will start reading again all the goals.

        If there's the case where the goal doesn't exist as a goal in the
        rules then it will ask the user (via ask_about()) for the existence
        of it. If user is not sure about it then the algorithm ends.

  mixed()
    As its name says, it's a mix of forward() and backward() algorithms, it
    requires to have at least one initial fact.

    The first thing it does is to run the forward() algorithm (hence the
    need of at least one initial fact). If the algorithm fails then the
    mixed algorithm also ends unsuccessfully.

    Once the first *run* of forward() algorithm happens it starts looking
    for any positive inference fact, if only one is found then this ends the
    algorithm with the assumption it knows what's happening.

    In case no positive inference fact is found then it will start reading
    the rules and creating a list of intuitive facts.

    For each rule it will get a *certainty factor* of its causes versus the
    initial, inference and asked facts. In case the certainity factor is
    greater or equal than found_factor then all of its goals will be copied
    to the intuitive facts (eg, read it as: it assumes the goals have
    something to do with our first initial facts).

    Once all the rules are read then it verifies if there are intuitive
    facts, if no facts are found then it ends with the intuition, otherwise
    it will run the backward() algorithm for each one of these facts (eg,
    each fact will be converted to a goal). After each *run* of the
    backward() algorithm it will verify for any positive inference fact, if
    just one is found then the algorithm ends.

    At the end (if there are still no positive inference facts) it will run
    the forward() algorithm and restart (by looking again for any positive
    inference fact).

    A good example to understand how this algorithm is useful is: imagine
    you are a doctor and know some of the symptoms of a patient. Probably
    with the first symptoms you have you can get to a positive conclusion
    (eg that a patient has *X* disease). However in case there's still no
    clue, then a set of questions (done by the call of backward()) of
    symptons related to the initial symptoms will be asked to the user. For
    example, we know that that the patient has a headache but that doesn't
    give us any positive answer, what if the patient has flu or another
    disease? Then a set of these *related* symptons will be asked to the
    user.

  summary($return)
    The main purpose of any expert system is the ability to explain: what is
    happening, how it got to a result, what assumption(s) it required to
    make, the fatcs that were excluded and the ones that were used.

    This method will use the viewer (or return the result) in YAML format of
    all the rules that were shot. It will explain how it got to each one of
    the causes so a better explanation can be done by the viewer.

    If $return is defined (eg, it got any parameter) then the result wont be
    passed to the viewer, instead it will be returned as a string.

SEE ALSO
    Take a look AI::ExpertSystem::Simple too.

AUTHOR
    Pablo Fischer (pablo@pablo.com.mx).

COPYRIGHT
    Copyright (C) 2010 by Pablo Fischer.

    This library is free software; you can redistribute it and/or modify it
    under the same terms as Perl itself.