Backward-chaining inference engine.
Usage: ./expert_system filename filename - path to input file.
Input file format: First block: Lines, that contains rules. Vocabulary: XOR - ^, OR - |, AND - +, NOT - !, Implies - =>, If and only if - <=>. Second block: Single line, starting with '=' - Set of initial facts. All other are false. Third block: Single line, startig with '?' - Facts, which values you want to obtain.