sealed abstract class Terminator() {
  override def toString = this match {
    case Jump(b)         => "jump " + b.id
    case Cond(c, b1, b2) => "cond " + b1.id + "/" + b2.id
    case Return(v)       => "retn"
  }
}

case class Jump(val target: BasicBlock) extends Terminator()
case class Cond(val cond: Value, val arm1: BasicBlock, val arm2: BasicBlock) extends Terminator() with Use
case class Return(val value: Value) extends Terminator() with Use