diff --git a/project/build/XSbt.scala b/project/build/XSbt.scala
index 843898188..f714f5d53 100644
--- a/project/build/XSbt.scala
+++ b/project/build/XSbt.scala
@@ -102,6 +102,7 @@ class XSbt(info: ProjectInfo) extends ParentProject(info) with NoCrossPaths
 	{
 		override def scratch = true
 		override def consoleClasspath = testClasspath
+		override def compileOptions = super.compileOptions ++ compileOptions("-Xelide-below", "3000")
 	}
 	trait Licensed extends BasicScalaProject
 	{
diff --git a/tasks/CompletionService.scala b/tasks/CompletionService.scala
new file mode 100644
index 000000000..9dc924df4
--- /dev/null
+++ b/tasks/CompletionService.scala
@@ -0,0 +1,44 @@
+/* sbt -- Simple Build Tool
+ * Copyright 2010 Mark Harrah
+ */
+package sbt
+
+trait CompletionService[A, R]
+{
+	def submit(node: A,  work: () => R): () => R
+	def take(): R
+}
+
+import java.util.concurrent.{Callable, CompletionService => JCompletionService, Executor, Executors, ExecutorCompletionService}
+
+object CompletionService
+{
+	def apply[A, T](poolSize: Int): (CompletionService[A,T], () => Unit) =
+	{
+		val pool = Executors.newFixedThreadPool(2)
+		(apply[A,T]( pool ), () => pool.shutdownNow() )
+	}
+	def apply[A, T](x: Executor): CompletionService[A,T] =
+		apply(new ExecutorCompletionService[T](x))
+	def apply[A, T](completion: JCompletionService[T]): CompletionService[A,T] =
+		new CompletionService[A, T] {
+			def submit(node: A,  work: () => T) = {
+				val future = completion.submit { new Callable[T] { def call = work() } }
+				() => future.get()
+			}
+			def take() = completion.take().get()
+		}
+
+	def manage[A, T](service: CompletionService[A,T])(setup: A => Unit, cleanup: A => Unit): CompletionService[A,T] =
+		wrap(service) { (node, work) => () =>
+			setup(node)
+			try { work() }
+			finally { cleanup(node) }
+		}
+	def wrap[A, T](service: CompletionService[A,T])(w: (A, () => T) => (() => T)): CompletionService[A,T] =
+		new CompletionService[A,T]
+		{
+			def submit(node: A, work: () => T) = service.submit(node, w(node, work))
+			def take() = service.take()
+		}
+}
\ No newline at end of file
diff --git a/tasks/Execute.scala b/tasks/Execute.scala
new file mode 100644
index 000000000..5fb362a10
--- /dev/null
+++ b/tasks/Execute.scala
@@ -0,0 +1,318 @@
+/* sbt -- Simple Build Tool
+ * Copyright 2010 Mark Harrah
+ */
+package sbt
+
+// TODO: Incomplete needs to be parameterized with A[_] and have val node
+
+import Node._
+import Types._
+import Execute._
+
+import scala.annotation.tailrec
+import scala.collection.{mutable, JavaConversions}
+import mutable.Map
+
+object Execute
+{
+	trait Part1of2K[M[_[_], _], A[_]] { type Apply[T] = M[A, T] }
+	type NodeT[A[_]] = Part1of2K[Node, A]
+	
+	def idMap[A,B]: Map[A, B] = JavaConversions.asMap(new java.util.IdentityHashMap[A,B])
+	def pMap[A[_], B[_]]: PMap[A,B] = new DelegatingPMap[A, B](idMap)
+	private[sbt] def completed(p: => Unit): Completed = new Completed {
+		def process() { p }
+	}
+}
+sealed trait Completed {
+	def process(): Unit
+}
+
+
+final class Execute[A[_] <: AnyRef](checkCycles: Boolean)(implicit view: A ~> NodeT[A]#Apply )
+{
+	type Strategy = CompletionService[A[_], Completed]
+
+	private[this] val forward = idMap[A[_], IDSet[A[_]] ]
+	private[this] val reverse = idMap[A[_], Iterable[A[_]] ]
+	private[this] val callers = pMap[A, Compose[IDSet,A]#Apply ]
+	private[this] val state = idMap[A[_], State]
+	private[this] val viewCache = pMap[A, NodeT[A]#Apply]
+	private[this] val results = pMap[A, Result]
+
+	private[this] type State = State.Value
+	private[this] object State extends Enumeration {
+		val Pending, Running, Calling, Done = Value
+	}
+	import State.{Pending, Running, Calling, Done}
+
+	def dump: String = "State: "  + state.toString + "\n\nResults: " + results + "\n\nCalls: " + callers + "\n\n"
+
+	def run[T](root: A[T])(implicit strategy: Strategy) =
+	{
+		assert(state.isEmpty, "Execute already running/ran.")
+
+		addNew(root)
+		processAll()
+		assert( results contains root, "No result for root node." )
+		results(root)
+	}
+
+	def processAll()(implicit strategy: Strategy)
+	{
+		@tailrec def next()
+		{
+			pre {
+				assert( !reverse.isEmpty, "Nothing to process." )
+				assert( state.values.exists( _ == Running ), "Nothing running")
+			}
+
+			(strategy.take()).process()
+			if( !reverse.isEmpty ) next()
+		}
+		next()
+
+		post {
+			assert( reverse.isEmpty, "Did not process everything." )
+			assert( complete, "Not all state was Done." )
+		}
+	}
+
+
+	def call[T](node: A[T], target: A[T])(implicit strategy: Strategy)
+	{
+		if(checkCycles) cycleCheck(node, target)
+		pre {
+			assert( running(node) )
+			readyInv( node )
+		}
+
+		state(node) = Calling
+		addChecked(target)
+		addCaller(node, target)
+
+		post {
+			assert( calling(node) )
+			assert( callers(target) contains node )
+			readyInv( node )
+		}
+	}
+
+	def retire[T](node: A[T], result: Result[T])(implicit strategy: Strategy)
+	{
+		pre {
+			assert( running(node) | calling(node) )
+			readyInv( node )
+		}
+
+		results(node) = result
+		state(node) = Done
+		remove( reverse, node ) foreach { dep => notifyDone(node, dep) }
+		callers.remove( node ).flatten.foreach { c =>  retire(c, result) }
+
+		post {
+			assert( done(node) )
+			assert( results(node) == result )
+			readyInv( node )
+			assert( ! (reverse contains node) )
+			assert( ! (callers contains node) )
+		}
+	}
+
+	def notifyDone( node: A[_], dependent: A[_] )(implicit strategy: Strategy)
+	{
+		val f = forward(dependent)
+		f -= node
+		if( f.isEmpty ) {
+			remove(forward, dependent)
+			ready( dependent )
+		}
+	}
+	/** Ensures the given node has been added to the system.
+	* Once added, a node is pending until its inputs and dependencies have completed.
+	* Its computation is then evaluated and made available for nodes that have it as an input.*/
+	def addChecked[T](node: A[T])(implicit strategy: Strategy)
+	{
+		if( !added(node)) addNew(node)
+
+		post { addedInv( node ) }
+	}
+	/** Adds a node that has not yet been registered with the system.
+	* If all of the node's dependencies have finished, the node's computation scheduled to run.
+	* The node's dependencies will be added (transitively) if they are not already registered.
+	* */
+	def addNew[T](node: A[T])(implicit strategy: Strategy)
+	{
+		pre { newPre(node) }
+
+		val v = register( node )
+		val deps = dependencies(v)
+		val active = deps filter notDone
+
+		if( active.isEmpty)
+			ready( node )
+		else
+		{
+			forward(node) = IDSet(active)
+			val newD = active filter isNew
+			newD foreach { x => addNew(x) }
+			active foreach { addReverse(_, node) }
+		}
+
+		post {
+			addedInv( node )
+			assert( running(node) ^ pending(node) )
+			if( running(node) ) runningInv( node )
+			if( pending(node) ) pendingInv( node )
+		}
+	}
+	/** Called when a pending 'node' becomes runnable.  All of its dependencies must be done.  This schedules the node's computation with 'strategy'.*/
+	def ready[T]( node: A[T] )(implicit strategy: Strategy)
+	{
+		pre {
+			assert( pending(node) )
+			readyInv( node )
+			assert( reverse contains node )
+		}
+
+		state(node) = Running
+		submit(node)
+
+		post {
+			readyInv( node )
+			assert( reverse contains node )
+			assert( running( node ) )
+		}
+	}
+	/** Enters the given node into the system. */
+	def register[T](node: A[T]): Node[A, T] = 
+	{
+		state(node) = Pending
+		reverse(node) = Seq()
+		viewCache.getOrUpdate(node, view(node))
+	}
+	def incomplete(in: A[_]): Option[(A[_], Incomplete)] =
+		results(in) match {
+			case Value(v) => None
+			case Inc(inc) => Some( (in, inc) )
+		}
+		/** Send the work for this node to the provided Strategy. */
+	def submit[T]( node: A[T] )(implicit strategy: Strategy)
+	{
+		val v = view(node)
+		val rs: v.Inputs#Map[Result] = v.inputs.map(results)
+		val ud = v.unitDependencies.flatMap(incomplete)
+		strategy.submit( node, () => work(node, v.work(rs, ud)) )
+	}
+	/** Evaluates the computation 'f' for 'node'.
+	* This returns a Completed instance, which contains the post-processing to perform after the result is retrieved from the Strategy.*/
+	def work[T](node: A[T], f: => Either[A[T], T])(implicit strategy: Strategy): Completed =
+	{
+		val result =
+			try { Right(f) }
+			catch {
+				case i: Incomplete => Left(i)
+				case e => Left( Incomplete(Incomplete.Error, directCause = Some(e)) )
+			}
+		completed {
+			result match {
+				case Left(i) => retire(node, Inc(i))
+				case Right(Right(v)) => retire(node, Value(v))
+				case Right(Left(target)) => call(node, target)
+			}
+		}
+	}
+
+	def remove[K, V](map: Map[K, V], k: K): V = map.remove(k).getOrElse(error("Key '" + k + "' not in map :\n" + map))
+
+	def addReverse(node: A[_], dependent: A[_]): Unit = reverse(node) ++= Seq(dependent)
+	def addCaller[T](caller: A[T], target: A[T]): Unit = callers.getOrUpdate(target, IDSet.create[A[T]]) += caller
+
+	def dependencies(node: A[_]): Iterable[A[_]] = dependencies(view(node))
+	def dependencies(v: Node[A, _]): Iterable[A[_]] = v.unitDependencies ++ v.inputs.toList
+
+		// Contracts
+
+	def addedInv(node: A[_]): Unit = topologicalSort(node) foreach addedCheck
+	def addedCheck(node: A[_])
+	{
+		assert( added(node), "Not added: " + node )
+		assert( viewCache contains node, "Not in view cache: " + node )
+		dependencyCheck( node )
+	}
+	def dependencyCheck( node: A[_] )
+	{
+		dependencies( node ) foreach { dep =>
+			def onOpt[T](o: Option[T])(f: T => Boolean) = o match { case None => false; case Some(x) => f(x) }
+			def checkForward = onOpt( forward.get(node) ) { _ contains dep }
+			def checkReverse = onOpt( reverse.get(dep) ){ _.toSet contains node }
+			assert( done(dep) ^ ( checkForward && checkReverse ) )
+		}
+	}
+	def pendingInv(node: A[_])
+	{
+		assert( atState(node, Pending) )
+		assert( dependencies( node ) exists notDone )
+	}
+	def runningInv( node: A[_] )
+	{
+		assert( dependencies( node ) forall done )
+		assert( ! (forward contains node) )
+	}
+	def newPre(node: A[_])
+	{
+		isNew(node)
+		assert(!(reverse contains node))
+		assert(!(forward contains node))
+		assert(!(callers contains node))
+		assert(!(viewCache contains node))
+		assert(!(results contains node))
+	}
+
+	def topologicalSort(node: A[_]): Seq[A[_]] =
+	{
+		val seen = IDSet.create[A[_]]
+		def visit(n: A[_]): List[A[_]] =
+			(seen process n)( List[A[_]]() ) {
+				node :: (List[A[_]]() /: dependencies(n) ) { (ss, dep) => visit(dep) ::: ss}
+			}
+
+		visit(node).reverse
+	}
+
+	def readyInv(node: A[_])
+	{
+		assert( dependencies(node) forall done )
+		assert( ! ( forward contains node ) )
+	}
+
+		// cyclic reference checking
+
+	def cycleCheck[T](node: A[T], target: A[T])
+	{
+		if(node eq target) cyclic(node, target, "Cannot call self")
+		val all = IDSet.create[A[T]]
+		def allCallers(n: A[T]): Unit = (all process n)(()) { callers.get(n).flatten.foreach(allCallers) }
+		allCallers(node)
+		if(all contains target) cyclic(node, target, "Cyclic reference")
+	}
+	def cyclic[T](caller: A[T], target: A[T], msg: String) = throw new Incomplete(message = Some(msg), directCause = Some( new CyclicException(caller, target, msg) ) )
+	final class CyclicException[T](val caller: A[T], val target: A[T], msg: String) extends Exception(msg)
+
+		// state testing
+
+	def pending(d: A[_]) = atState(d, Pending)
+	def running(d: A[_]) = atState(d, Running)
+	def calling(d: A[_]) = atState(d, Calling)
+	def done(d: A[_]) = atState(d, Done)
+	def notDone(d: A[_]) = !done(d)
+	def atState(d: A[_], s: State) = state.get(d) == Some(s)
+	def isNew(d: A[_]) = !added(d)
+	def added(d: A[_]) = state contains d
+	def complete = state.values.forall(_ == Done)
+
+	import scala.annotation.elidable
+	import elidable._
+	@elidable(ASSERTION) def pre(f: => Unit) = f
+	@elidable(ASSERTION) def post(f: => Unit) = f
+}
diff --git a/tasks/Incomplete.scala b/tasks/Incomplete.scala
index 9ac78e556..06b551892 100644
--- a/tasks/Incomplete.scala
+++ b/tasks/Incomplete.scala
@@ -3,4 +3,10 @@
  */
 package sbt
 
-trait Incomplete
\ No newline at end of file
+import Incomplete.{Error, Value => IValue}
+final case class Incomplete(tpe: IValue = Error, message: Option[String] = None, causes: Seq[Incomplete] = Nil, directCause: Option[Throwable] = None)
+	extends Exception(message.orNull, directCause.orNull)
+
+object Incomplete extends Enumeration {
+	val Skipped, Error = Value
+}
\ No newline at end of file
diff --git a/tasks/Node.scala b/tasks/Node.scala
index 90a5cd6e6..b57c5dc00 100644
--- a/tasks/Node.scala
+++ b/tasks/Node.scala
@@ -9,27 +9,27 @@ import Types._
 trait Node[A[_], T]
 {
 	type Inputs <: MList[A]
-	type Results <: Inputs#Map[Result]
+	type Results = Inputs#Map[Result]
 
 	val inputs: Inputs
 	def unitDependencies: Iterable[A[_]]
 
-	def work(results: Results, units: Iterable[Incomplete]): Either[A[T], T]
+	def work(results: Results, units: UnitResults[A]): Either[A[T], T]
 }
 
 object Node
 {
-	def pure[T](f: => T): PureNode[T]= map[Id, T, MNil](MNil, Nil)((_,_) => f)
+	/*def pure[T](f: () => T): PureNode[T]= map[Id, T, MNil](MNil, Nil)((_,_) => f() )
 
-	def map[A[_], T, Inputs0 <: MList[A]](inputs0: Inputs0, deps0: Iterable[A[_]])(work0: (Inputs0#Map[Result], Iterable[Incomplete]) => T):
-		Node[A,T] { type Inputs = Inputs0; type Results = Inputs0#Map[Result] } =
+	def map[A[_], T, Inputs0 <: MList[A]](inputs0: Inputs0, deps0: Iterable[A[_]])(work0: (Inputs0#Map[Result], UnitResults[A]) => T):
+		Node[A,T] { type Inputs = Inputs0 } =
 			new Node[A,T] {
 				type Inputs = Inputs0
-				type Results = Inputs0#Map[Result]
 				val inputs = inputs0
 				def unitDependencies = deps0
-				def work(results: Results, units: Iterable[Incomplete]) = Right(work0(results, units))
+				def work(results: Results, units: UnitResults[A]) = Right(work0(results, units))
 			}
 
-	type PureNode[T] = Node[Id, T] { type Inputs = MNil; type Results = MNil }
+	type PureNode[T] = Node[Id, T] { type Inputs = MNil; type Results = MNil }*/
+	type UnitResults[A[_]] = Iterable[(A[_], Incomplete)]
 }
diff --git a/tasks/Result.scala b/tasks/Result.scala
index e4fad1e14..326b9e66a 100644
--- a/tasks/Result.scala
+++ b/tasks/Result.scala
@@ -3,7 +3,23 @@
  */
 package sbt
 
-// used instead of Either[Throwable, T] for type inference
+// used instead of Either[Incomplete, T] for type inference
+
+/** Result of completely evaluating a task.*/
 sealed trait Result[+T]
-final case class Exc(cause: Throwable) extends Result[Nothing]
-final case class Value[+T](value: T) extends Result[T]
\ No newline at end of file
+/** Indicates the task did not complete normally and so it does not have a value.*/
+final case class Inc(cause: Incomplete) extends Result[Nothing]
+/** Indicates the task completed normally and produced the given `value`.*/
+final case class Value[+T](value: T) extends Result[T]
+
+object Result
+{
+	type Id[X] = X
+	val tryValue = new (Result ~> Id) {
+		def apply[T](r: Result[T]): T =
+			r match {
+				case Value(v) => v
+				case Inc(i) => throw i
+			}
+	}
+}
\ No newline at end of file
diff --git a/tasks/Test.scala b/tasks/Test.scala
new file mode 100644
index 000000000..8929685ae
--- /dev/null
+++ b/tasks/Test.scala
@@ -0,0 +1,114 @@
+/* sbt -- Simple Build Tool
+ * Copyright 2010 Mark Harrah
+ */
+package sbt
+
+import Types._
+import Node._
+import Task._
+import Execute._
+
+sealed trait Task[+T]
+sealed case class Pure[+T](eval: () => T) extends Task[T]
+sealed case class Mapped[+T, In <: MList[Task]](in: In, f: In#Map[Result] => T) extends Task[T]
+sealed case class MapAll[+T, In <: MList[Task]](in: In, f: In#Map[Result]#Raw => T) extends Task[T]
+sealed case class FlatMapAll[+T, In <: MList[Task]](in: In, f: In#Map[Result]#Raw => Task[T]) extends Task[T]
+sealed case class FlatMapped[+T, In <: MList[Task]](in: In, f: In#Map[Result] => Task[T]) extends Task[T]
+
+object Task
+{
+	implicit val taskToNode = new (Task ~> NodeT[Task]#Apply) {
+		def apply[T](t: Task[T]): Node[Task, T] = t match {
+			case Pure(eval) => toNode[T, MNil](MNil, _ => Right(eval()) )
+			case Mapped(in, f) => toNode[T, in.type](in, right ∙ f  )
+			case MapAll(in, f) => toNode[T, in.type](in, right ∙ (f compose all) )
+			case FlatMapAll(in, f) => toNode[T, in.type](in, left ∙ (f compose all) )
+			case FlatMapped(in, f) => toNode[T, in.type](in, left ∙ f )
+		}
+	}
+	def toNode[T, In <: MList[Task]](in: In, f: In#Map[Result] => Either[Task[T], T]): Node[Task, T] = new Node[Task, T] {
+		type Inputs = In
+		val inputs = in
+		def unitDependencies = Nil
+		def work(results: Results, units: UnitResults[Task]) = f(results)
+	}
+
+	def pure[T](name: String)(f: => T): Pure[T] = new Pure(f _) { override def toString = name }
+	def mapped[T, In0 <: MList[Task]](name: String)(in0: In0)(f0: In0#Map[Result] => T): Mapped[T, In0] = new Mapped(in0, f0) { override def toString = name }
+	def flat[T, In0 <: MList[Task]](name: String)(in0: In0)(f0: In0#Map[Result] => Task[T]): FlatMapped[T, In0] = new FlatMapped(in0, f0) {  override def toString = name }
+	def mapAll[T, In0 <: MList[Task]](name: String)(in0: In0)(f0: In0#Map[Result]#Raw => T): MapAll[T, In0] = new MapAll(in0, f0) {  override def toString = name }
+	def flatAll[T, In0 <: MList[Task]](name: String)(in0: In0)(f0: In0#Map[Result]#Raw => Task[T]): FlatMapAll[T, In0] = new FlatMapAll(in0, f0) {  override def toString = name }
+
+	def all[In <: MList[Result]]: In => In#Raw = in =>
+	{
+		val incs = in.toList.collect { case Inc(i) => i }
+		if(incs.isEmpty) in.down(Result.tryValue) else throw Incomplete(causes = incs)
+	}
+}
+object Test
+{
+	val a = pure("a")(3)
+	val b = pure[Boolean]("b")(error("test"))
+	val b2 = pure("b2")(true)
+	val c = pure("x")("asdf")
+	val i3 = a :^: b :^: c :^: MNil
+	val i32 = a :^: b2 :^: c :^: MNil
+
+	val fh= (_: Int :+: Boolean :+: String :+: HNil) match
+		{ case aa :+: bb :+: cc :+: HNil => aa + " " + bb + " " + cc }
+	val h1 = mapAll("h1")(i3)(fh)
+	val h2 = mapAll("h2")(i32)(fh)
+
+	val f: i3.Map[Result] => Any = {
+		case Value(aa) :^: Value(bb) :^: Value(cc) :^: MNil => aa + " " + bb + " " + cc
+		case x =>
+			val cs = x.toList.collect { case Inc(x) => x } // workaround for double definition bug
+			throw Incomplete(causes = cs)
+	}
+	val d2 = mapped("d2")(i32)(f)
+	val f2: i3.Map[Result] => Task[Any] = {
+		case Value(aa) :^: Value(bb) :^: Value(cc) :^: MNil => new Pure(() => aa + " " + bb + " " + cc)
+		case x => d3
+	}
+	lazy val d = flat("d")(i3)(f2)
+	val f3: i3.Map[Result] => Task[Any] = {
+		case Value(aa) :^: Value(bb) :^: Value(cc) :^: MNil => new Pure(() => aa + " " + bb + " " + cc)
+		case x => d2
+	}
+	lazy val d3= flat("d3")(i3)(f3)
+
+	def d4(i: Int): Task[Int] = flat("d4")(MNil){ _ => val x = math.random; if(x < 0.01) pure(x.toString)(i); else d4(i+1) }
+
+	lazy val pureEval =
+		new (Pure ~> Result) {
+			def apply[T](p: Pure[T]): Result[T] =
+				try { Value(p.eval()) }
+				catch { case e: Exception => throw Incomplete(Incomplete.Error, directCause = Some(e)) }
+		}
+
+	lazy val resultA = d.f( d.in.map(pureEval) )
+
+	def execute[T](root: Task[T]) = {
+		val (service, shutdown) = CompletionService[Task[_], Completed](2)
+		implicit val wrapped = CompletionService.manage(service)(x => println("Starting: " + x), x => println("Finished: " + x) )
+
+		val x = new Execute[Task](true)(taskToNode)
+		try { x.run(root) } finally { shutdown(); println(x.dump) }
+	}
+
+	def go()
+	{
+		def run[T](root: Task[T]) = 
+			println("Result : " + execute(root))
+	
+		run(a)
+		run(b)
+		run(b2)
+		run(c)
+		run(d)
+		run(d2)
+		run( d4(0) )
+		run(h1)
+		run(h2)
+	}
+}
\ No newline at end of file
diff --git a/util/collection/IDSet.scala b/util/collection/IDSet.scala
new file mode 100644
index 000000000..29ecf469d
--- /dev/null
+++ b/util/collection/IDSet.scala
@@ -0,0 +1,45 @@
+/* sbt -- Simple Build Tool
+ * Copyright 2010 Mark Harrah
+ */
+package sbt
+
+/** A mutable set interface that uses object identity to test for set membership.*/
+trait IDSet[T]
+{
+	def apply(t: T): Boolean
+	def contains(t: T): Boolean
+	def += (t: T): Unit
+	def ++=(t: Iterable[T]): Unit
+	def -= (t: T): Boolean
+	def all: collection.Iterable[T]
+	def isEmpty: Boolean
+	def foreach(f: T => Unit): Unit
+	def process[S](t: T)(ifSeen: S)(ifNew: => S): S
+}
+
+object IDSet
+{
+	implicit def toTraversable[T]: IDSet[T] => Traversable[T] = _.all
+	def apply[T](values: T*): IDSet[T] = apply(values)
+	def apply[T](values: Iterable[T]): IDSet[T] =
+	{
+		val s = create[T]
+		s ++= values
+		s
+	}
+	def create[T]: IDSet[T] = new IDSet[T] {
+		private[this] val backing = new java.util.IdentityHashMap[T, AnyRef]
+		private[this] val Dummy: AnyRef = ""
+
+		def apply(t: T) = contains(t)
+		def contains(t: T) = backing.containsKey(t)
+		def foreach(f: T => Unit) = all foreach f
+		def += (t: T) = backing.put(t, Dummy)
+		def ++=(t: Iterable[T]) = t foreach +=
+		def -= (t:T) = if(backing.remove(t) eq null) false else true
+		def all = collection.JavaConversions.asIterable(backing.keySet)
+		def isEmpty = backing.isEmpty
+		def process[S](t: T)(ifSeen: S)(ifNew: => S) = if(contains(t)) ifSeen else { this += t ; ifNew }
+		override def toString = backing.toString
+	}
+}
\ No newline at end of file
diff --git a/util/collection/MList.scala b/util/collection/MList.scala
index 58d2724c3..b350858c3 100644
--- a/util/collection/MList.scala
+++ b/util/collection/MList.scala
@@ -7,6 +7,11 @@ import Types._
 
 sealed trait MList[+M[_]]
 {
+	// For converting MList[Id] to an HList
+	//  This is useful because type inference doesn't work well with Id
+	type Raw <: HList
+	def down(implicit ev: M ~> Id): Raw
+
 	type Map[N[_]] <: MList[N]
 	def map[N[_]](f: M ~> N): Map[N]
 
@@ -14,6 +19,9 @@ sealed trait MList[+M[_]]
 }
 final case class MCons[H, +T <: MList[M], +M[_]](head: M[H], tail: T) extends MList[M]
 {
+	type Raw = H :+: tail.Raw
+	def down(implicit f: M ~> Id): Raw = HCons(f(head), tail.down(f))
+
 	type Map[N[_]] = MCons[H, tail.Map[N], N]
 	def map[N[_]](f: M ~> N) = MCons( f(head), tail.map(f) )
 
@@ -23,6 +31,9 @@ final case class MCons[H, +T <: MList[M], +M[_]](head: M[H], tail: T) extends ML
 }
 sealed class MNil extends MList[Nothing]
 {
+	type Raw = HNil
+	def down(implicit f: Nothing ~> Id) = HNil
+
 	type Map[N[_]] = MNil
 	def map[N[_]](f: Nothing ~> N) = MNil
 
@@ -30,4 +41,4 @@ sealed class MNil extends MList[Nothing]
 
 	def toList = Nil
 }
-object MNil extends MNil
+object MNil extends MNil
\ No newline at end of file
diff --git a/util/collection/PMap.scala b/util/collection/PMap.scala
new file mode 100644
index 000000000..bc5e092af
--- /dev/null
+++ b/util/collection/PMap.scala
@@ -0,0 +1,43 @@
+/* sbt -- Simple Build Tool
+ * Copyright 2010 Mark Harrah
+ */
+package sbt
+
+import Types._
+
+trait PMap[K[_], V[_]] extends (K ~> V)
+{
+	def apply[T](k: K[T]): V[T]
+	def get[T](k: K[T]): Option[V[T]]
+	def update[T](k: K[T], v: V[T]): Unit
+	def contains[T](k: K[T]): Boolean
+	def remove[T](k: K[T]): Option[V[T]]
+	def getOrUpdate[T](k: K[T], make: => V[T]): V[T]
+}
+object PMap
+{
+	implicit def toFunction[K[_], V[_]](map: PMap[K,V]): K[_] => V[_] = k => map(k)
+}
+
+abstract class AbstractPMap[K[_], V[_]] extends PMap[K,V]
+{
+	def apply[T](k: K[T]): V[T] = get(k).get
+	def contains[T](k: K[T]): Boolean = get(k).isDefined
+}
+
+import collection.mutable.Map
+
+/** Only suitable for K that is invariant in its parameter.
+* Option and List keys are not, for example, because None <:< Option[String] and None <: Option[Int].*/
+class DelegatingPMap[K[_], V[_]](backing: Map[K[_], V[_]]) extends AbstractPMap[K,V]
+{
+	def get[T](k: K[T]): Option[V[T]] = cast[T]( backing.get(k) )
+	def update[T](k: K[T], v: V[T]) { backing(k) = v }
+	def remove[T](k: K[T]) = cast( backing.remove(k) )
+	def getOrUpdate[T](k: K[T], make: => V[T]) = cast[T]( backing.getOrElseUpdate(k, make) )
+
+	private[this] def cast[T](v: V[_]): V[T] = v.asInstanceOf[V[T]]
+	private[this] def cast[T](o: Option[V[_]]): Option[V[T]] = o map cast[T]
+
+	override def toString = backing.toString
+}
\ No newline at end of file
diff --git a/util/collection/Param.scala b/util/collection/Param.scala
new file mode 100644
index 000000000..3271465d9
--- /dev/null
+++ b/util/collection/Param.scala
@@ -0,0 +1,31 @@
+/* sbt -- Simple Build Tool
+ * Copyright 2010 Mark Harrah
+ */
+package sbt
+
+import Types._
+
+// Used to emulate ~> literals
+trait Param[A[_], B[_]]
+{
+	type T
+	def in: A[T]
+	def ret(out: B[T])
+	def ret: B[T]
+}
+
+object Param
+{
+	implicit def pToT[A[_], B[_]](p: Param[A,B] => Unit): A~>B = new (A ~> B) {
+		def apply[s](a: A[s]): B[s] = {
+			val v: Param[A,B] { type T = s} = new Param[A,B] { type T = s
+				def in = a
+				private var r: B[T] = _
+				def ret(b: B[T]) {r = b}
+				def ret: B[T] = r
+			}
+			p(v)
+			v.ret
+		}
+	}
+}
\ No newline at end of file
diff --git a/util/collection/Rewrite.scala b/util/collection/Rewrite.scala
new file mode 100644
index 000000000..40b40add4
--- /dev/null
+++ b/util/collection/Rewrite.scala
@@ -0,0 +1,42 @@
+/* sbt -- Simple Build Tool
+ * Copyright 2010 Mark Harrah
+ */
+package sbt
+
+import Types._
+
+trait Rewrite[A[_]]
+{
+	def apply[T](node: A[T], rewrite: Rewrite[A]): A[T]
+}
+object Rewrite
+{
+	def Id[A[_]]: Rewrite[A] = new Rewrite[A] { def apply[T](node: A[T], rewrite: Rewrite[A]) = node }
+
+	implicit def specificF[T](f: T => T): Rewrite[Const[T]#Apply] = new Rewrite[Const[T]#Apply] {
+		def apply[S](node:T, rewrite: Rewrite[Const[T]#Apply]): T = f(node)
+	}
+	implicit def pToRewrite[A[_]](p: Param[A,A] => Unit): Rewrite[A] = toRewrite(Param.pToT(p))
+	implicit def toRewrite[A[_]](f: A ~> A): Rewrite[A] = new Rewrite[A] {
+		def apply[T](node: A[T], rewrite:Rewrite[A]) = f(node)
+	}
+	def compose[A[_]](a: Rewrite[A], b: Rewrite[A]): Rewrite[A] =
+		new Rewrite[A] {
+			def apply[T](node: A[T], rewrite: Rewrite[A]) =
+				a(b(node, rewrite), rewrite)
+		}
+	implicit def rewriteOps[A[_]](outer: Rewrite[A]): RewriteOps[A] =
+		new RewriteOps[A] {
+			def ∙(g: A ~> A): Rewrite[A] = compose(outer, g)
+			def andThen(g: A ~> A): Rewrite[A] = compose(g, outer)
+			def ∙(g: Rewrite[A]): Rewrite[A] = compose(outer, g)
+			def andThen(g: Rewrite[A]): Rewrite[A] = compose(g, outer)
+		}
+	def apply[A[_], T](value: A[T])(implicit rewrite: Rewrite[A]): A[T] = rewrite(value, rewrite)
+}
+trait RewriteOps[A[_]]
+{
+	def andThen(g: A ~> A): Rewrite[A]
+	def ∙(g: A ~> A): Rewrite[A]
+	def ∙(g: Rewrite[A]): Rewrite[A]
+}
\ No newline at end of file
diff --git a/util/collection/TypeFunctions.scala b/util/collection/TypeFunctions.scala
index 95fb8ae9e..00a6fc772 100644
--- a/util/collection/TypeFunctions.scala
+++ b/util/collection/TypeFunctions.scala
@@ -7,14 +7,26 @@ trait TypeFunctions
 {
 	type Id[X] = X
 	trait Const[A] { type Apply[B] = A }
-	trait P1of2[M[_,_], A] { type Apply[B] = M[A,B] }
-	trait Down[M[_]] { type Apply[B] = Id[M[B]] }
+	trait Compose[A[_], B[_]] { type Apply[T] = A[B[T]] }
+	trait P1of2[M[_,_], A] { type Apply[B] = M[A,B]; type Flip[B] = M[B, A] }
 
-	trait ~>[-A[_], +B[_]]
-	{
-		def apply[T](a: A[T]): B[T]
-	}
-	def Id: Id ~> Id =
-		new (Id ~> Id) { def apply[T](a: T): T = a }
+	final val left = new (Id ~> P1of2[Left, Nothing]#Flip) { def apply[T](t: T) = Left(t) }
+	final val right = new (Id ~> P1of2[Right, Nothing]#Apply) { def apply[T](t: T) = Right(t) }
+	final val some = new (Id ~> Some) { def apply[T](t: T) = Some(t) }
 }
-object TypeFunctions extends TypeFunctions
\ No newline at end of file
+object TypeFunctions extends TypeFunctions
+
+trait ~>[-A[_], +B[_]]
+{ outer =>
+	def apply[T](a: A[T]): B[T]
+	// directly on ~> because of type inference limitations
+	final def ∙[C[_]](g: C ~> A): C ~> B = new (C ~> B) { def apply[T](c: C[T]) = outer.apply(g(c)) }
+	final def ∙[C,D](g: C => D)(implicit ev: D <:< A[D]): C => B[D] = i => apply(ev(g(i)) )
+	final def fn[T] = (t: A[T]) => apply[T](t)
+}
+object ~>
+{
+	import TypeFunctions._
+	val Id: Id ~> Id = new (Id ~> Id) { def apply[T](a: T): T = a }
+	implicit def tcIdEquals: (Id ~> Id) = Id
+}
\ No newline at end of file
diff --git a/util/collection/Types.scala b/util/collection/Types.scala
index de468216d..de6cf5aec 100644
--- a/util/collection/Types.scala
+++ b/util/collection/Types.scala
@@ -8,4 +8,4 @@ object Types extends TypeFunctions
 	val :^: = MCons
 	val :+: = HCons
 	type :+:[H, T <: HList] = HCons[H,T]
-}
\ No newline at end of file
+}
diff --git a/util/collection/src/test/scala/LiteralTest.scala b/util/collection/src/test/scala/LiteralTest.scala
new file mode 100644
index 000000000..76fffe80a
--- /dev/null
+++ b/util/collection/src/test/scala/LiteralTest.scala
@@ -0,0 +1,17 @@
+/* sbt -- Simple Build Tool
+ * Copyright 2010 Mark Harrah
+ */
+package sbt
+
+import Types._
+
+// compilation test
+object LiteralTest {
+  def x[A[_],B[_]](f: A ~> B) = f
+
+  import Param._
+  val f = x { (p: Param[Option,List]) => p.ret( p.in.toList ) }
+
+  val a: List[Int] = f( Some(3) )
+  val b: List[String] = f( Some("aa") )
+}
\ No newline at end of file
diff --git a/util/collection/src/test/scala/MListTest.scala b/util/collection/src/test/scala/MListTest.scala
new file mode 100644
index 000000000..ffb86b18e
--- /dev/null
+++ b/util/collection/src/test/scala/MListTest.scala
@@ -0,0 +1,19 @@
+/* sbt -- Simple Build Tool
+ * Copyright 2010 Mark Harrah
+ */
+package sbt
+
+import Types._
+
+object MTest {
+  val f = new (Option ~> List) { def apply[T](o: Option[T]): List[T] = o.toList }
+
+  val x = Some(3) :^: Some("asdf") :^: MNil
+  val y = x map f
+  val m1a = y match { case List(3) :^: List("asdf") :^: MNil => println("true") }
+  val m1b = (List(3) :^: MNil)  match { case yy :^: MNil => println("true") }
+
+  val head = new (List ~> Id) { def apply[T](xs: List[T]): T = xs.head }
+  val z = y.map[Id](head).down
+  val m2 = z match { case 3 :+: "asdf" :+: HNil => println("true") }
+}
diff --git a/util/collection/src/test/scala/PMapTest.scala b/util/collection/src/test/scala/PMapTest.scala
new file mode 100644
index 000000000..1ea66daaa
--- /dev/null
+++ b/util/collection/src/test/scala/PMapTest.scala
@@ -0,0 +1,18 @@
+/* sbt -- Simple Build Tool
+ * Copyright 2010 Mark Harrah
+ */
+package sbt
+
+import Types._
+
+// compilation test
+object PMapTest
+{
+	val mp = new DelegatingPMap[Some, Id](new collection.mutable.HashMap)
+	mp(Some("asdf")) = "a"
+	mp(Some(3)) = 9
+	val x = Some(3) :^: Some("asdf") :^: MNil
+	val y = x.map[Id](mp)
+	val z = y.down
+	z match { case 9 :+: "a" :+: HNil => println("true") }
+}
\ No newline at end of file
diff --git a/util/collection/src/test/scala/RewriteTest.scala b/util/collection/src/test/scala/RewriteTest.scala
new file mode 100644
index 000000000..c2ca1b237
--- /dev/null
+++ b/util/collection/src/test/scala/RewriteTest.scala
@@ -0,0 +1,50 @@
+/* sbt -- Simple Build Tool
+ * Copyright 2010 Mark Harrah
+ */
+package sbt
+
+import Types._
+
+object RewriteTest
+{
+	// dist and add0 show the awkwardness when not just manipulating superstructure:
+	//  would have to constrain the parameters to Term to be instances of Zero/Eq somehow
+	val dist: Rewrite[Term] = (p: Param[Term, Term]) => p.ret( p.in match {
+		case Add(Mult(a,b),Mult(c,d)) if a == c=> Mult(a, Add(b,d))
+		case x => x
+	})
+	val add0: Rewrite[Term] = (p: Param[Term, Term]) => p.ret( p.in match {
+		case Add(V(0), y) => y
+		case Add(x, V(0)) => x
+		case x => x
+	})
+	val rewriteBU= new Rewrite[Term] {
+		def apply[T](node: Term[T], rewrite: Rewrite[Term]) = {
+			def r[T](node: Term[T]) = rewrite(node, rewrite)
+			node match {
+				case Add(x, y) => Add(r(x), r(y))
+				case Mult(x, y) => Mult(r(x), r(y))
+				case x => x
+			}
+		}
+	}
+
+	val d2 = dist ∙ add0 ∙ rewriteBU
+
+	implicit def toV(t: Int): V[Int] = V(t)
+	implicit def toVar(s: String): Var[Int] = Var[Int](s)
+
+	val t1: Term[Int] = Add(Mult(3,4), Mult(4, 5))
+	val t2: Term[Int] = Add(Mult(4,4), Mult(4, 5))
+	val t3: Term[Int] = Add(Mult(Add("x", 0),4), Mult("x", 5))
+
+	println( Rewrite(t1)(d2) )
+	println( Rewrite(t2)(d2) )
+	println( Rewrite(t3)(d2) )
+}
+
+sealed trait Term[T]
+final case class Add[T](a: Term[T], b: Term[T]) extends Term[T]
+final case class Mult[T](a: Term[T], b: Term[T]) extends Term[T]
+final case class V[T](v: T) extends Term[T]
+final case class Var[T](name: String) extends Term[T]