* [FORMAL_LANGUAGE,NEW] Added add_individual_constant/2, add_predicate_tuple/2.
* [FORMAL_LANGUAGE,NEW] Added logical_connective/[1,2].
* [FORMAL_LANGUAGE,NEW] Added wrapper individual_constant/1.
* [FORMAL_LANGUAGE,NEW] Added operator_depth/2 and reimplemented formula/1, using the underlying formula/2.
* [FORMAL_LANGUAGE,NEW] Added predicate_arity/2, replacing predicate/3.
* [FORMAL_LANGUAGE,EXT] Added various exceptions throwing.
* [FORMAL_LANGUAGE,DOC] Improced documentation of predicates.
* [FORMAL_LANGUAGE,DEB] Added unit tests for atomic_formula/1, subformula/2.
* [FORMAL_LANGUAGE,NEW] Added subformula/2.