diff --git a/summingbird-core-test/src/test/scala/com/twitter/summingbird/planner/DagOptimizerTest.scala b/summingbird-core-test/src/test/scala/com/twitter/summingbird/planner/DagOptimizerTest.scala
new file mode 100644
index 000000000..36073bb06
--- /dev/null
+++ b/summingbird-core-test/src/test/scala/com/twitter/summingbird/planner/DagOptimizerTest.scala
@@ -0,0 +1,153 @@
+package com.twitter.summingbird.planner
+
+import com.twitter.algebird.Semigroup
+import com.twitter.summingbird._
+import com.twitter.summingbird.graph.{DependantGraph, Rule}
+import com.twitter.summingbird.memory._
+
+import org.scalatest.FunSuite
+import org.scalacheck.{Arbitrary, Gen}
+import Gen.oneOf
+
+import scala.collection.mutable
+import org.scalatest.prop.GeneratorDrivenPropertyChecks._
+
+class DagOptimizerTest extends FunSuite {
+
+  implicit val generatorDrivenConfig =
+    PropertyCheckConfig(minSuccessful = 1000, maxDiscarded = 1000) // the producer generator uses filter, I think
+    //PropertyCheckConfig(minSuccessful = 100, maxDiscarded = 1000) // the producer generator uses filter, I think
+
+  import TestGraphGenerators._
+  import MemoryArbitraries._
+  implicit def testStore: Memory#Store[Int, Int] = mutable.Map[Int, Int]()
+  implicit def testService: Memory#Service[Int, Int] = new mutable.HashMap[Int, Int]() with MemoryService[Int, Int]
+  implicit def sink1: Memory#Sink[Int] = ((_) => Unit)
+  implicit def sink2: Memory#Sink[(Int, Int)] = ((_) => Unit)
+
+  def genProducer: Gen[Producer[Memory, _]] = oneOf(genProd1, genProd2, summed)
+
+  test("DagOptimizer round trips") {
+    forAll { p: Producer[Memory, Int] =>
+      val dagOpt = new DagOptimizer[Memory] { }
+
+      assert(dagOpt.toLiteral(p).evaluate == p)
+    }
+  }
+
+  val dagOpt = new DagOptimizer[Memory] { }
+
+  test("ExpressionDag fanOut matches DependantGraph") {
+    forAll(genProducer) { p: Producer[Memory, _] =>
+      val expDag = dagOpt.expressionDag(p)._1
+
+      // the expression considers an also a fanout, so
+      // we can't use the standard Dependants, we need to
+      // us parentsOf as the edge function
+      val deps = new DependantGraph[Producer[Memory, Any]] {
+        override lazy val nodes: List[Producer[Memory, Any]] = Producer.entireGraphOf(p)
+        override def dependenciesOf(p: Producer[Memory, Any]) = Producer.parentsOf(p)
+      }
+
+      deps.nodes.foreach { n =>
+        deps.fanOut(n) match {
+          case Some(fo) => assert(expDag.fanOut(n) == fo)
+          case None => fail(s"node $n has no fanOut value")
+        }
+      }
+    }
+  }
+
+
+  val allRules = {
+    import dagOpt._
+
+    List(RemoveNames,
+      RemoveIdentityKeyed,
+      FlatMapFusion,
+      OptionMapFusion,
+      OptionToFlatMap,
+      KeyFlatMapToFlatMap,
+      FlatMapKeyFusion,
+      ValueFlatMapToFlatMap,
+      FlatMapValuesFusion,
+      FlatThenOptionFusion,
+      DiamondToFlatMap,
+      MergePullUp,
+      AlsoPullUp)
+  }
+
+  val genRule: Gen[Rule[dagOpt.Prod]] =
+    for {
+      n <- Gen.choose(1, allRules.size)
+      rs <- Gen.pick(n, allRules) // get n randomly selected
+    } yield rs.reduce(_.orElse(_))
+
+  test("Rules are idempotent") {
+    forAll(genProducer, genRule) { (p, r) =>
+      val once = dagOpt.optimize(p, r)
+      val twice = dagOpt.optimize(once, r)
+      assert(once == twice)
+    }
+  }
+
+  test("fanOut matches after optimization") {
+
+    forAll(genProducer, genRule) { (p, r) =>
+
+      val once = dagOpt.optimize(p, r)
+
+      val expDag = dagOpt.expressionDag(once)._1
+      // the expression considers an also a fanout, so
+      // we can't use the standard Dependants, we need to
+      // us parentsOf as the edge function
+      val deps = new DependantGraph[Producer[Memory, Any]] {
+        override lazy val nodes: List[Producer[Memory, Any]] = Producer.entireGraphOf(once)
+        override def dependenciesOf(p: Producer[Memory, Any]) = Producer.parentsOf(p)
+      }
+
+      deps.nodes.foreach { n =>
+        deps.fanOut(n) match {
+          case Some(fo) => assert(expDag.fanOut(n) == fo, s"node: $n, in optimized: $once")
+          case None => fail(s"node $n has no fanOut value")
+        }
+      }
+    }
+
+  }
+
+  test("test some idempotency specific past failures") {
+    val list = List(-483916215)
+    val list2 = list
+
+    val map1 = new MemoryService[Int, Int] {
+      val map = Map(1122506458 -> -422595330)
+      def get(i: Int) = map.get(i)
+    }
+
+    val fn1 = { (i: Int) => List((i, i)) }
+    val fn2 = { (tup: (Int, Int)) => Option(tup) }
+    val fn3 = { i: (Int, (Int, Option[Int])) => List((i._1, i._2._1)) }
+    val fn4 = fn1
+    val fn5 = fn2
+    val mmap: Memory#Store[Int, Int] = collection.mutable.Map.empty[Int, Int]
+
+    val arg0: Producer[Memory, (Int, (Option[Int], Int))] =
+      Summer[Memory, Int, Int](IdentityKeyedProducer(NamedProducer(IdentityKeyedProducer(MergedProducer(IdentityKeyedProducer(FlatMappedProducer(LeftJoinedProducer[Memory, Int, Int, Int](IdentityKeyedProducer(NamedProducer(IdentityKeyedProducer(NamedProducer(IdentityKeyedProducer(OptionMappedProducer(IdentityKeyedProducer(FlatMappedProducer(Source[Memory, Int](list), fn1)), fn2)),"tjiposzOlkplcu")),"tvpwpdyScehGnwcaVjjWvlfuwxatxhdjhozscucpbq")), map1), fn3)),IdentityKeyedProducer(OptionMappedProducer(IdentityKeyedProducer(FlatMappedProducer(Source[Memory, Int](list2), fn4)),fn5)))),"ncn")),mmap, implicitly[Semigroup[Int]])
+
+    val dagOpt = new DagOptimizer[Memory] { }
+
+    val rule = {
+      import dagOpt._
+      List[Rule[Prod]](
+        RemoveNames,
+        RemoveIdentityKeyed,
+        FlatMapFusion,
+        OptionToFlatMap
+        ).reduce(_ orElse _)
+    }
+    val once = dagOpt.optimize(arg0, rule)
+    val twice = dagOpt.optimize(once, rule)
+    assert(twice == once)
+  }
+}
diff --git a/summingbird-core/src/main/scala/com/twitter/summingbird/graph/ExpressionDag.scala b/summingbird-core/src/main/scala/com/twitter/summingbird/graph/ExpressionDag.scala
index d97216578..a76763aac 100644
--- a/summingbird-core/src/main/scala/com/twitter/summingbird/graph/ExpressionDag.scala
+++ b/summingbird-core/src/main/scala/com/twitter/summingbird/graph/ExpressionDag.scala
@@ -126,6 +126,7 @@ sealed trait ExpressionDag[N[_]] { self =>
         }
       }
       // Note this Stream must always be non-empty as long as roots are
+      // TODO: we don't need to use collect here, just .get on each id in s
       idToExp.collect[IdSet](partial)
         .reduce(_ ++ _)
     }
@@ -162,12 +163,6 @@ sealed trait ExpressionDag[N[_]] { self =>
     curr
   }
 
-  protected def toExpr[T](n: N[T]): (ExpressionDag[N], Expr[T, N]) = {
-    val (dag, id) = ensure(n)
-    val exp = dag.idToExp(id)
-    (dag, exp)
-  }
-
   /**
    * Convert a N[T] to a Literal[T, N]
    */
@@ -182,10 +177,19 @@ sealed trait ExpressionDag[N[_]] { self =>
       def apply[U] = {
         val fn = rule.apply[U](self)
 
+        def ruleApplies(id: Id[U]): Boolean = {
+          val n = evaluate(id)
+          fn(n) match {
+            case Some(n1) => n != n1
+            case None => false
+          }
+        }
+
+
         {
-          case (id, exp) if fn(exp.evaluate(idToExp)).isDefined =>
+          case (id, _) if ruleApplies(id) =>
             // Sucks to have to call fn, twice, but oh well
-            (id, fn(exp.evaluate(idToExp)).get)
+            (id, fn(evaluate(id)).get)
         }
       }
     }
@@ -193,21 +197,37 @@ sealed trait ExpressionDag[N[_]] { self =>
       case None => this
       case Some(tup) =>
         // some type hand holding
-        def act[T](in: HMap[Id, N]#Pair[T]) = {
+        def act[T](in: HMap[Id, N]#Pair[T]): ExpressionDag[N] = {
+          /*
+           * We can't delete Ids which may have been shared
+           * publicly, and the ids may be embedded in many
+           * nodes. Instead we remap this i to be a pointer
+           * to the newid.
+           */
           val (i, n) = in
-          val oldNode = evaluate(i)
-          val (dag, exp) = toExpr(n)
-          dag.copy(id2Exp = dag.idToExp + (i -> exp))
+          val (dag, newId) = ensure(n)
+          dag.copy(id2Exp = dag.idToExp + (i -> Var[T, N](newId)))
         }
         // This cast should not be needed
         act(tup.asInstanceOf[HMap[Id, N]#Pair[Any]]).gc
     }
   }
 
-  // This is only called by ensure
+  /**
+   * This is only called by ensure
+   *
+   * Note, Expr must never be a Var
+   */
   private def addExp[T](node: N[T], exp: Expr[T, N]): (ExpressionDag[N], Id[T]) = {
-    val nodeId = Id[T](nextId)
-    (copy(id2Exp = idToExp + (nodeId -> exp), id = nextId + 1), nodeId)
+    require(!exp.isInstanceOf[Var[T, N]])
+
+    find(node) match {
+      case None =>
+        val nodeId = Id[T](nextId)
+        (copy(id2Exp = idToExp + (nodeId -> exp), id = nextId + 1), nodeId)
+      case Some(id) =>
+        (this, id)
+    }
   }
 
   /**
@@ -216,9 +236,18 @@ sealed trait ExpressionDag[N[_]] { self =>
    */
   def find[T](node: N[T]): Option[Id[T]] = nodeToId.getOrElseUpdate(node, {
     val partial = new GenPartial[HMap[Id, E]#Pair, Id] {
-      def apply[T1] = { case (thisId, expr) if node == expr.evaluate(idToExp) => thisId }
+      def apply[T1] = {
+        // Make sure to return the original Id, not a Id -> Var -> Expr
+        case (thisId, expr) if !expr.isInstanceOf[Var[_, N]] && node == expr.evaluate(idToExp) => thisId
+      }
+    }
+    idToExp.collect(partial).toList match {
+      case Nil => None
+      case id :: Nil =>
+        // this cast is safe if node == expr.evaluate(idToExp) implies types match
+        Some(id).asInstanceOf[Option[Id[T]]]
+      case others => None//sys.error(s"logic error, should only be one mapping: $node -> $others")
     }
-    idToExp.collect(partial).headOption.asInstanceOf[Option[Id[T]]]
   })
 
   /**
@@ -247,7 +276,7 @@ sealed trait ExpressionDag[N[_]] { self =>
              * Since the code is not performance critical, but correctness critical, and we can't
              * check this property with the typesystem easily, check it here
              */
-            assert(n == node,
+            require(n == node,
               "Equality or nodeToLiteral is incorrect: nodeToLit(%s) = ConstLit(%s)".format(node, n))
             addExp(node, Const(n))
           case UnaryLit(prev, fn) =>
@@ -272,10 +301,7 @@ sealed trait ExpressionDag[N[_]] { self =>
 
   def evaluateOption[T](id: Id[T]): Option[N[T]] =
     idToN.getOrElseUpdate(id, {
-      val partial = new GenPartial[HMap[Id, E]#Pair, N] {
-        def apply[T1] = { case (thisId, expr) if (id == thisId) => expr.evaluate(idToExp) }
-      }
-      idToExp.collect(partial).headOption.asInstanceOf[Option[N[T]]]
+      idToExp.get(id).map(_.evaluate(idToExp))
     })
 
   /**
@@ -284,25 +310,31 @@ sealed trait ExpressionDag[N[_]] { self =>
    * We need to garbage collect nodes that are
    * no longer reachable from the root
    */
-  def fanOut(id: Id[_]): Int = {
-    // We make a fake IntT[T] which is just Int
-    val partial = new GenPartial[E, ({ type IntT[T] = Int })#IntT] {
-      def apply[T] = {
-        case Var(id1) if (id1 == id) => 1
-        case Unary(id1, fn) if (id1 == id) => 1
-        case Binary(id1, id2, fn) if (id1 == id) && (id2 == id) => 2
-        case Binary(id1, id2, fn) if (id1 == id) || (id2 == id) => 1
-        case _ => 0
-      }
-    }
-    idToExp.collectValues[({ type IntT[T] = Int })#IntT](partial).sum
+  def fanOut(id: Id[_]): Int =
+    evaluateOption(id)
+      .map(fanOut)
+      .getOrElse(0)
+
+  @annotation.tailrec
+  private def dependsOn(expr: Expr[_, N], node: N[_]): Boolean = expr match {
+    case Const(_) => false
+    case Var(id) => dependsOn(idToExp(id), node)
+    case Unary(id, _) => evaluate(id) == node
+    case Binary(id0, id1, _) => evaluate(id0) == node || evaluate(id1) == node
   }
 
   /**
    * Returns 0 if the node is absent, which is true
    * use .contains(n) to check for containment
    */
-  def fanOut(node: N[_]): Int = find(node).map(fanOut(_)).getOrElse(0)
+  def fanOut(node: N[_]): Int = {
+    val pointsToNode = new GenPartial[HMap[Id, E]#Pair, N] {
+      def apply[T] = {
+        case (id, expr) if dependsOn(expr, node) => evaluate(id)
+      }
+    }
+    idToExp.collect[N](pointsToNode).toSet.size
+  }
   def contains(node: N[_]): Boolean = find(node).isDefined
 }
 
@@ -355,6 +387,9 @@ trait Rule[N[_]] { self =>
     def apply[T](on: ExpressionDag[N]) = { n =>
       self.apply(on)(n).orElse(that.apply(on)(n))
     }
+
+    override def toString: String =
+      s"$self.orElse($that)"
   }
 }
 
diff --git a/summingbird-core/src/main/scala/com/twitter/summingbird/graph/package.scala b/summingbird-core/src/main/scala/com/twitter/summingbird/graph/package.scala
index 60da67d68..33c62a0ed 100644
--- a/summingbird-core/src/main/scala/com/twitter/summingbird/graph/package.scala
+++ b/summingbird-core/src/main/scala/com/twitter/summingbird/graph/package.scala
@@ -56,7 +56,8 @@ package object graph {
         }
       }
       // make sure the values are sets, not .mapValues is lazy in scala
-      .map { case (k, v) => (k, v.distinct) };
+      .map { case (k, v) => (k, v.distinct) }
+
     graph.getOrElse(_, Nil)
   }
 
diff --git a/summingbird-core/src/test/scala/com/twitter/summingbird/graph/ExpressionDagTests.scala b/summingbird-core/src/test/scala/com/twitter/summingbird/graph/ExpressionDagTests.scala
index 2a32f0dbf..2e0db8363 100644
--- a/summingbird-core/src/test/scala/com/twitter/summingbird/graph/ExpressionDagTests.scala
+++ b/summingbird-core/src/test/scala/com/twitter/summingbird/graph/ExpressionDagTests.scala
@@ -177,6 +177,7 @@ object ExpressionDagTests extends Properties("ExpressionDag") {
   } yield Inc(chain, by)
 
   def genChain: Gen[Formula[Int]] = Gen.frequency((1, genConst), (3, genChainInc))
+
   property("CombineInc compresses linear Inc chains") = forAll(genChain) { chain =>
     ExpressionDag.applyRule(chain, toLiteral, CombineInc) match {
       case Constant(n) => true