Moved "instance set:...{power} from Set.thy to RelPow.thy, where is is needed

--- a/src/HOL/RelPow.thy	Fri Oct 30 10:43:12 1998 +0100
+++ b/src/HOL/RelPow.thy	Fri Oct 30 10:45:08 1998 +0100
@@ -8,6 +8,9 @@
 
 RelPow = Nat +
 
+instance
+  set :: (term) {power}   (* only ('a * 'a) set should be in power! *)
+
 primrec
   "R^0 = Id"
   "R^(Suc n) = R O (R^n)"

--- a/src/HOL/Set.thy	Fri Oct 30 10:43:12 1998 +0100
+++ b/src/HOL/Set.thy	Fri Oct 30 10:45:08 1998 +0100
@@ -18,7 +18,7 @@
   set :: (term) term
 
 instance
-  set :: (term) {ord, minus, power} (* only ('a * 'a) set should be in power! *)
+  set :: (term) {ord, minus}
 
 syntax
   "op :"        :: ['a, 'a set] => bool             ("op :")