OWL DL extension question

DLers,

Was wondering if anyone is looking into whether this fragment can be
added tractably to OWL DL (1.0):

  (forall (x y z)
     (if (and (P x y)
              (R x z))
         (S y z)))

For example, the uncles of my children include all my brothers
(P=children, R=brothers, S=uncles).

My understanding is this is different from general role inclusion:

  (forall (x y z)
     (if (and (P x y)
              (R y z))
         (S x z)))

For example, my brothers include all the uncles of my children
(P=children, R=uncles, S=brothers).

Conrad