DEFINITION papp()
TYPE =
       PList→PList→PList
BODY =
FIXpapp{
         papp:PList→PList→PList
         :=λa:PList
           .λb:PList
             .<λp:PList.PList> CASE a OF PNil⇒b | PCons h d a0⇒PCons h d (papp a0 b)
         }