why not conformant?

object hom {
  trait set {
    type inf
    type sup >: inf
    type hom[_>:inf<:sup,_>:inf<:sup]
    type dual = set.of[inf,sup,({type λ[x>:inf<:sup,y>:inf<:sup]=hom[y,x]})#λ]
  }
  object set {
    trait of[l,h>:l,c[_>:l<:h,_>:l<:h]] extends set {
      type inf = l
      type sup = h
      type hom[x>:inf<:sup,y>:inf<:sup] = c[x,y]
    }
    trait singleton[z] extends of[Nothing,Nothing,({type λ[x>:Nothing<:Nothing,y>:Nothing<:Nothing]=z})#λ]
  }
}

trait category[h <: hom.set] { 
  def id[A>:h#inf<:h#sup]: h#hom[A,A]
  def compose[A>:h#inf<:h#sup, B>:h#inf<:h#sup, C>:h#inf<:h#sup](f : h#hom[B,C], g: h#hom[A,B]) : h#hom[A,C]
  type dual = category[h#dual]
  def dual : dual 
}



<console>:9: error: type arguments [y,x] do not conform to type hom's type parameter bounds [_ >: set.this.inf <: set.this.sup,_ >: set.this.inf <: set.this.sup]
         type dual = category[h#dual]