invalidity of “subject-obversion” rule

in contemporary texts of islamic logic, the “subject-obversion” has been introduced as a rule on basis of which one can replace the subject term of a categorical proposition with its complement and change both quality and quantity of the proposition. thus from an a-proposition such as “all a is b”, one can infer “some not-a is not b”. well, let’s take that a is included by b but not vice versa, and b is most extensive universal such as “existent” or “thing”. in this case “all a is b” produces the contradictory of our favorite conclusion, i.e. “all not-a is b”. therefore, the so-called subject-obversion rule, at least in a-proposition, is not logically valid.