(Feature Introduction) there is a type Intro(F) ∈ TYPE such that:  A type signature is a structure TYPE, , FEAT, Approp , where TYPE, is a type hierarchy, FEAT is a finite set of features, FEAT and TYPE are disjoint, and Approp is an appropriateness specification.
in S 4 , {q 2 , q 3 } ≈ {q 4 } and {q 5 } ≈ {q 6 }, and after coalescing these two pairs, the result is S 5 which is non-redundant.