[scala-language] Seq[Set].flatten => error: could not find implicit value for parameter asTraversable ?

Hello,

I just found that I'm not able to do a flatten on a Seq of Set, and that
seems really strange as I'm able to flatten a Seq of Iterables.

Is it related to http://lampsvn.epfl.ch/trac/scala/ticket/3813 ?
If not, could someone explain me what is the reason for that ?

8<---------- REPL session ----------------

Welcome to Scala version 2.8.1.final (Java HotSpot(TM) Server VM, Java
1.6.0_23).
Type in expressions to have them evaluated.
Type :help for more information.

scala> Seq( Set("a"), Set(), Set("b", "a") ).flatten
:6: error: could not find implicit value for parameter
asTraversable: (scala.collection.immutable.Set[_ <: java.lang.String])
=> Traversable[B]
Seq( Set("a"), Set(), Set("b", "a") ).flatten
^

scala> Seq( Seq("a"), Seq(), Seq("b", "a") ).flatten
res1: Seq[java.lang.String] = List(a, b, a)

scala> Set( Seq("a"), Seq(), Seq("b", "a") ).flatten
res2: scala.collection.immutable.Set[java.lang.String] = Set(a, b)

scala> Seq( Seq("a"), Seq(), Seq("b", "a") ).flatMap( _.toSeq )
res3: Seq[java.lang.String] = List(a, b, a)

Seq( Iterable("a"), Iterable("a", "b") ).flatten
res4: Seq[java.lang.String] = List(a, a, b)

Seq( Set("a"):Iterable[String], Set():Iterable[String], Set("b",
"a"):Iterable[String] ).flatten
res5: Seq[String] = List(a, b, a)

8<---------- /REPL session ----------------

Thanks,