The husk and stray are counted as sub-variants of existing mobs as opposed to entirely new mobs, hence why you are unable to simply type /summon Stray or /summon Husk. There was supposed to be spawn eggs for them, but it seems they were removed either intentionally or by accident in the final release.
Axiomatic method, in logic, a procedure by which an entire system (e.g., a science) is generated in accordance with specified rules by logical deduction from certain basic propositions (axioms or postulates), which in turn are constructed from a few terms taken as primitive.
Mathematical logic is often used in proof theory, set theory, model theory, and recursion theory. Proof theory is, quite logically, the study of formal proofs. Sets of propositions can be used to conclude new relationships. Set theory studies 'sets,' which are collections of objects.