diff --git a/util/collection/Relation.scala b/util/collection/Relation.scala
index b305bb47c..c5195ffb7 100644
--- a/util/collection/Relation.scala
+++ b/util/collection/Relation.scala
@@ -29,7 +29,7 @@ object Relation
 	private[sbt] def combine[X,Y](a: M[X,Y], b: M[X,Y]): M[X,Y] =
 		(a /: b) { (map, mapping) => add(map, mapping._1, mapping._2) }
 
-	private[sbt] def add[X,Y](map: M[X,Y], from: X, to: Iterable[Y]): M[X,Y] =
+	private[sbt] def add[X,Y](map: M[X,Y], from: X, to: Traversable[Y]): M[X,Y] =
 		map.updated(from,  get(map, from) ++ to)
 
 	private[sbt] def get[X,Y](map: M[X,Y], t: X): Set[Y] = map.getOrElse(t, Set.empty[Y])
@@ -49,15 +49,15 @@ trait Relation[A,B]
 	/** Includes the relation (a, b). */
 	def +(a: A, b: B): Relation[A,B]
 	/** Includes the relations (a, b) for all b in bs. */
-	def +(a: A, bs: Iterable[B]): Relation[A,B]
+	def +(a: A, bs: Traversable[B]): Relation[A,B]
 	/** Returns the union of the relation r with this relation. */
 	def ++(r: Relation[A,B]): Relation[A,B]
 	/** Includes the given relations. */
-	def ++(rs: Iterable[(A,B)]): Relation[A,B]
+	def ++(rs: Traversable[(A,B)]): Relation[A,B]
 	/** Removes all relations (_1, _2) for all _1 in _1s. */
-	def --(_1s: Iterable[A]): Relation[A,B]
+	def --(_1s: Traversable[A]): Relation[A,B]
 	/** Removes all `pairs` from this relation. */
-	def --(pairs: Traversable[(A,B)]): Relation[A,B]
+	def --(pairs: TraversableOnce[(A,B)]): Relation[A,B]
 	/** Removes all pairs (_1, _2) from this relation. */
 	def -(_1: A): Relation[A,B]
 	/** Removes `pair` from this relation. */
@@ -69,6 +69,11 @@ trait Relation[A,B]
 	/** Returns the number of pairs in this relation */
 	def size: Int
 
+	/** Returns true iff (a,b) is in this relation*/
+	def contains(a: A, b: B): Boolean
+	/** Returns a relation with only pairs (a,b) for which f(a,b) is true.*/
+	def filter(f: (A,B) => Boolean): Relation[A,B]
+	
 	/** Returns all pairs in this relation.*/
 	def all: Traversable[(A,B)]
 	
@@ -92,14 +97,14 @@ private final class MRelation[A,B](fwd: Map[A, Set[B]], rev: Map[B, Set[A]]) ext
 	
 	def +(pair: (A,B)) = this + (pair._1, Set(pair._2))
 	def +(from: A, to: B) = this + (from, to :: Nil)
-	def +(from: A, to: Iterable[B]) =
+	def +(from: A, to: Traversable[B]) =
 		new MRelation( add(fwd, from, to), (rev /: to) { (map, t) => add(map, t, from :: Nil) })
 
-	def ++(rs: Iterable[(A,B)]) = ((this: Relation[A,B]) /: rs) { _ + _ }
+	def ++(rs: Traversable[(A,B)]) = ((this: Relation[A,B]) /: rs) { _ + _ }
 	def ++(other: Relation[A,B]) = new MRelation[A,B]( combine(fwd, other.forwardMap), combine(rev, other.reverseMap) )
 
-	def --(ts: Iterable[A]): Relation[A,B] = ((this: Relation[A,B]) /: ts) { _ - _ }
-	def --(pairs: Traversable[(A,B)]): Relation[A,B] = ((this: Relation[A,B]) /: pairs) { _ - _ }
+	def --(ts: Traversable[A]): Relation[A,B] = ((this: Relation[A,B]) /: ts) { _ - _ }
+	def --(pairs: TraversableOnce[(A,B)]): Relation[A,B] = ((this: Relation[A,B]) /: pairs) { _ - _ }
 	def -(pair: (A,B)): Relation[A,B] =
 		new MRelation( remove(fwd, pair._1, pair._2), remove(rev, pair._2, pair._1) )
 	def -(t: A): Relation[A,B] =
@@ -110,5 +115,9 @@ private final class MRelation[A,B](fwd: Map[A, Set[B]], rev: Map[B, Set[A]]) ext
 			case None => this
 		}
 
+	def filter(f: (A,B) => Boolean): Relation[A,B] = Relation.empty[A,B] ++ all.filter(f.tupled)
+
+	def contains(a: A, b: B): Boolean = forward(a)(b)
+
 	override def toString = all.map { case (a,b) => a + " -> " + b }.mkString("Relation [", ", ", "]")
 }
\ No newline at end of file