diff --git a/tests/neg/variances1.scala b/tests/neg/variances1.scala
new file mode 100644
index 000000000000..aabfd32565e9
--- /dev/null
+++ b/tests/neg/variances1.scala
@@ -0,0 +1,8 @@
+import scala.reflect.ClassTag
+class Foo[+A: ClassTag](x: A) {
+
+  private[this] val elems: Array[A] = Array(x)
+
+  def f[B](x: Array[B] = elems): Array[B] = x // error (1) should give a variance error here or ...
+
+}
\ No newline at end of file
diff --git a/tests/run/Pouring2.check b/tests/run/Pouring2.check
new file mode 100644
index 000000000000..f07f29105c0b
--- /dev/null
+++ b/tests/run/Pouring2.check
@@ -0,0 +1,2 @@
+Vector(Empty(0), Empty(1), Fill(0), Fill(1), Pour(0,1), Pour(1,0))
+Fill(1) Pour(1,0) Empty(0) Pour(1,0) Fill(1) Pour(1,0) --> Vector(4, 6)
diff --git a/tests/run/Pouring2.scala b/tests/run/Pouring2.scala
new file mode 100644
index 000000000000..d0d5241431da
--- /dev/null
+++ b/tests/run/Pouring2.scala
@@ -0,0 +1,62 @@
+class Pouring(capacity: Vector[Int]) {
+  type Glass = Int
+  type Content = Vector[Int]
+
+  sealed trait Move {
+    import Move._
+    def apply(content: Content): Content = this match {
+      case Empty(g) => content.updated(g, 0)
+      case Fill(g) => content.updated(g, capacity(g))
+      case Pour(from, to) =>
+        val amount = content(from) min (capacity(to) - content(to))
+        def adjust(s: Content, g: Glass, delta: Int) = s.updated(g, s(g) + delta)
+        adjust(adjust(content, from, -amount), to, amount)
+    }
+  }
+  object Move {
+    case class Empty(glass: Glass) extends Move
+    case class Fill(glass: Glass) extends Move
+    case class Pour(from: Glass, to: Glass) extends Move
+  }
+
+  val moves: Seq[Move] = {
+    val glasses = 0 until capacity.length
+    (for (g <- glasses) yield Move.Empty(g)) ++
+    (for (g <- glasses) yield Move.Fill(g)) ++
+    (for (g1 <- glasses; g2 <- glasses if g1 != g2) yield Move.Pour(g1, g2))
+  }
+
+  class Path(history: List[Move], val endContent: Content) {
+    def extend(move: Move) = new Path(move :: history, move(endContent))
+    override def toString = s"${history.reverse.mkString(" ")} --> $endContent"
+  }
+
+  val initialContent: Content = capacity.map(x => 0)
+  val initialPath = new Path(Nil, initialContent)
+
+  def from(paths: Set[Path], explored: Set[Content]): LazyList[Set[Path]] =
+    if (paths.isEmpty) LazyList.empty
+    else {
+      val extensions =
+        for {
+          path <- paths
+          move <- moves
+          next = path.extend(move)
+          if !explored.contains(next.endContent)
+        } yield next
+      paths #:: from(extensions, explored ++ extensions.map(_.endContent))
+    }
+
+  def solutions(target: Int): LazyList[Path] =
+    for {
+      paths <- from(Set(initialPath), Set(initialContent))
+      path <- paths
+      if path.endContent.contains(target)
+    } yield path
+}
+
+object Test extends App {
+  val problem = new Pouring(Vector(4, 7))
+  println(problem.moves)
+  println(problem.solutions(6).head)
+}
\ No newline at end of file
diff --git a/tests/run/Signals.check b/tests/run/Signals.check
new file mode 100644
index 000000000000..229f6b1ffb6b
--- /dev/null
+++ b/tests/run/Signals.check
@@ -0,0 +1,3 @@
+0
+10
+30
diff --git a/tests/run/Signals.scala b/tests/run/Signals.scala
new file mode 100644
index 000000000000..e320b579ad41
--- /dev/null
+++ b/tests/run/Signals.scala
@@ -0,0 +1,71 @@
+
+import annotation.unchecked._
+package frp with
+
+  sealed class Signal[+T](expr: (given Signal.Caller) => T) with
+    private var myExpr: Signal.Caller => T = _
+    private var myValue: T = _
+    private var observers: Set[Signal.Caller] = Set()
+    changeTo(expr)
+
+    protected def changeTo(expr: (given Signal.Caller) => T @uncheckedVariance): Unit =
+      myExpr = (caller => expr(given caller))
+      computeValue()
+
+    def apply()(given caller: Signal.Caller) =
+      observers += caller
+      assert(!caller.observers.contains(this), "cyclic signal definition")
+      myValue
+
+    protected def computeValue(): Unit =
+      val newValue = myExpr(this)
+      val observeChange = observers.nonEmpty && newValue != myValue
+      myValue = newValue
+      if observeChange then
+        val obs = observers
+        observers = Set()
+        obs.foreach(_.computeValue())
+
+  object Signal with
+    type Caller = Signal[?]
+    given noCaller: Caller(???) with
+      override def computeValue() = ()
+  end Signal
+
+  class Var[T](expr: (given Signal.Caller) => T) extends Signal[T](expr) with
+    def update(expr: (given Signal.Caller) => T): Unit = changeTo(expr)
+  end Var
+end frp
+
+import frp._
+class BankAccount with
+  def balance: Signal[Int] = myBalance
+
+  private var myBalance: Var[Int] = Var(0)
+
+  def deposit(amount: Int): Unit =
+    if amount > 0 then
+      val b = myBalance()
+      myBalance() = b + amount
+
+  def withdraw(amount: Int): Int =
+    if 0 < amount && amount <= balance() then
+      val b = myBalance()
+      myBalance() = b - amount
+      myBalance()
+    else assertFail("insufficient funds")
+end BankAccount
+
+@main def Test() =
+  def consolidated(accts: List[BankAccount]): Signal[Int] =
+    Signal(accts.map(_.balance()).sum)
+
+  val a = BankAccount()
+  val b = BankAccount()
+  val c = consolidated(List(a, b))
+  println(c())
+  a.deposit(10)
+  println(c())
+  b.deposit(20)
+  println(c())
+end Test
diff --git a/tests/run/Signals1.check b/tests/run/Signals1.check
new file mode 100644
index 000000000000..229f6b1ffb6b
--- /dev/null
+++ b/tests/run/Signals1.check
@@ -0,0 +1,3 @@
+0
+10
+30
diff --git a/tests/run/Signals1.scala b/tests/run/Signals1.scala
new file mode 100644
index 000000000000..0c461652af4e
--- /dev/null
+++ b/tests/run/Signals1.scala
@@ -0,0 +1,84 @@
+
+import annotation.unchecked._
+package frp with
+
+  trait Signal[+T] with
+    def apply()(given caller: Signal.Caller): T
+
+  object Signal with
+
+    abstract class AbstractSignal[+T] extends Signal[T] with
+      private var currentValue: T = _
+      private var observers: Set[Caller] = Set()
+
+      protected def eval: Caller => T
+
+      protected def computeValue(): Unit =
+        val newValue = eval(this)
+        val observeChange = observers.nonEmpty && newValue != currentValue
+        currentValue = newValue
+        if observeChange then
+          val obs = observers
+          observers = Set()
+          obs.foreach(_.computeValue())
+
+      def apply()(given caller: Caller): T =
+        observers += caller
+        assert(!caller.observers.contains(this), "cyclic signal definition")
+        currentValue
+    end AbstractSignal
+
+    def apply[T](expr: (given Caller) => T): Signal[T] =
+      new AbstractSignal[T] with
+        protected val eval = expr(given _)
+        computeValue()
+
+    class Var[T](expr: (given Caller) => T) extends AbstractSignal[T] with
+      protected var eval: Caller => T = expr(given _)
+      computeValue()
+
+      def update(expr: (given Caller) => T): Unit =
+        eval = expr(given _)
+        computeValue()
+    end Var
+
+    opaque type Caller = AbstractSignal[?]
+    given noCaller: Caller = new AbstractSignal[Nothing] with
+      override def eval = ???
+      override def computeValue() = ()
+
+  end Signal
+end frp
+
+import frp._
+class BankAccount with
+  def balance: Signal[Int] = myBalance
+
+  private val myBalance: Signal.Var[Int] = Signal.Var(0)
+
+  def deposit(amount: Int): Unit =
+    if amount > 0 then
+      val b = myBalance()
+      myBalance() = b + amount
+
+  def withdraw(amount: Int): Int =
+    if 0 < amount && amount <= balance() then
+      val b = myBalance()
+      myBalance() = b - amount
+      myBalance()
+    else assertFail("insufficient funds")
+end BankAccount
+
+@main def Test() =
+  def consolidated(accts: List[BankAccount]): Signal[Int] =
+    Signal(accts.map(_.balance()).sum)
+
+  val a = BankAccount()
+  val b = BankAccount()
+  val c = consolidated(List(a, b))
+  println(c())
+  a.deposit(10)
+  println(c())
+  b.deposit(20)
+  println(c())
+end Test
diff --git a/tests/run/instances.scala b/tests/run/instances.scala
index 482d7dc3ef24..aeb85d01d624 100644
--- a/tests/run/instances.scala
+++ b/tests/run/instances.scala
@@ -8,37 +8,34 @@ object Test extends App {
 
   case class Circle(x: Double, y: Double, radius: Double)
 
-  given circleOps: {
-    def (c: Circle) circumference: Double = c.radius * math.Pi * 2
-  }
+  given circleOps: extension (c: Circle) with
+    def circumference: Double = c.radius * math.Pi * 2
 
   val circle = new Circle(1, 1, 2.0)
 
   assert(circle.circumference == circleOps.circumference(circle))
 
-  given stringOps: {
-    def (xs: Seq[String]) longestStrings: Seq[String] = {
+  given stringOps: extension (xs: Seq[String]) with
+    def longestStrings: Seq[String] =
       val maxLength = xs.map(_.length).max
       xs.filter(_.length == maxLength)
-    }
-  }
+
   val names = List("hi", "hello", "world")
   assert(names.longestStrings == List("hello", "world"))
 
-  given seqOps: {
-    def [T](xs: Seq[T]) second = xs.tail.head
-  }
+  given extension [T](xs: Seq[T]) with
+    def second = xs.tail.head
 
   assert(names.longestStrings.second == "world")
 
-  given listListOps: {
-    def [T](xs: List[List[T]]) flattened = xs.foldLeft[List[T]](Nil)(_ ++ _)
-  }
+  given listListOps: extension [T](xs: List[List[T]]) with
+    def flattened = xs.foldLeft[List[T]](Nil)(_ ++ _)
 
-  // A right associative op
+  // A right associative op. Note: can't use given extension for this!
   given prepend: {
     def [T](x: T) :: (xs: Seq[T]) = x +: xs
   }
+
   val ss: Seq[Int] = List(1, 2, 3)
   val ss1 = 0 :: ss
   assert(ss1 == List(0, 1, 2, 3))
@@ -46,17 +43,15 @@ object Test extends App {
   assert(List(names, List("!")).flattened == names :+ "!")
   assert(Nil.flattened == Nil)
 
-  trait SemiGroup[T] {
+  trait SemiGroup[T] with
     def (x: T) combine (y: T): T
-  }
-  trait Monoid[T] extends SemiGroup[T] {
+
+  trait Monoid[T] extends SemiGroup[T] with
     def unit: T
-  }
 
-  given StringMonoid : Monoid[String] {
+  given StringMonoid : Monoid[String] with
     def (x: String) combine (y: String): String = x.concat(y)
     def unit: String = ""
-  }
 
   // Abstracting over a typeclass with a context bound:
   def sum[T: Monoid](xs: List[T]): T =
@@ -64,30 +59,28 @@ object Test extends App {
 
   println(sum(names))
 
-  trait Ord[T] {
+  trait Ord[T] with
     def (x: T) compareTo (y: T): Int
     def (x: T) < (y: T) = x.compareTo(y) < 0
     def (x: T) > (y: T) = x.compareTo(y) > 0
     val minimum: T
-  }
+  end Ord
 
-  given Ord[Int] {
+  given Ord[Int] with
     def (x: Int) compareTo (y: Int) =
       if (x < y) -1 else if (x > y) +1 else 0
     val minimum = Int.MinValue
-  }
 
-  given ListOrd[T: Ord] : Ord[List[T]] {
-    def (xs: List[T]) compareTo (ys: List[T]): Int = (xs, ys) match {
+  given listOrd[T: Ord]: Ord[List[T]] with
+    def (xs: List[T]) compareTo (ys: List[T]): Int = (xs, ys) match
       case (Nil, Nil) => 0
       case (Nil, _) => -1
       case (_, Nil) => +1
       case (x :: xs1, y :: ys1) =>
         val fst = x.compareTo(y)
         if (fst != 0) fst else xs1.compareTo(ys1)
-    }
     val minimum: List[T] = Nil
-  }
+  end listOrd
 
   def max[T: Ord](x: T, y: T): T = if (x < y) y else x
 
@@ -98,35 +91,33 @@ object Test extends App {
 
   println(max(List(1, 2, 3), List(2)))
 
-  trait Functor[F[_]] {
+  trait Functor[F[_]] with
     def [A, B](x: F[A]) map (f: A => B): F[B]
-  }
+  end Functor
 
-  trait Monad[F[_]] extends Functor[F] {
+  trait Monad[F[_]] extends Functor[F] with
     def [A, B](x: F[A]) flatMap (f: A => F[B]): F[B]
     def [A, B](x: F[A]) map (f: A => B) = x.flatMap(f `andThen` pure)
 
     def pure[A](x: A): F[A]
-  }
+  end Monad
 
-  given ListMonad : Monad[List] {
+  given listMonad: Monad[List] with
     def [A, B](xs: List[A]) flatMap (f: A => List[B]): List[B] =
       xs.flatMap(f)
     def pure[A](x: A): List[A] =
       List(x)
-  }
 
-  given ReaderMonad[Ctx] : Monad[[X] =>> Ctx => X] {
+  given readerMonad[Ctx]: Monad[[X] =>> Ctx => X] with
     def [A, B](r: Ctx => A) flatMap (f: A => Ctx => B): Ctx => B =
       ctx => f(r(ctx))(ctx)
     def pure[A](x: A): Ctx => A =
       ctx => x
-  }
 
-  def mappAll[F[_]: Monad, T](x: T, fs: List[T => T]): F[T] =
-    fs.foldLeft(implicitly[Monad[F]].pure(x))((x: F[T], f: T => T) =>
-      if (true) implicitly[Monad[F]].map(x)(f)
-      else if (true) x.map(f)
+  def mapAll[F[_]: Monad, T](x: T, fs: List[T => T]): F[T] =
+    fs.foldLeft(summon[Monad[F]].pure(x))((x: F[T], f: T => T) =>
+      if true then summon[Monad[F]].map(x)(f)
+      else if true then x.map(f)
       else x.map[T, T](f)
     )
 }
\ No newline at end of file