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.