def evalChained(expr: Expr, references: Map[String, Signal[Expr]], seen: Set[String]): Double = {
    expr match {
      case Literal(v) => v
      case Ref(name) => references.contains(name) match {
        case true => seen.contains(name) match {
          case false => evalChained(references(name)(), references, seen + name)
          case true => Double.NaN
        }
        case false => Double.NaN
      }
      case Plus(a, b) => evalChained(a, references, seen) + evalChained(b, references, seen)
      case Minus(a, b) => evalChained(a, references, seen) - evalChained(b, references, seen)
      case Times(a, b) => evalChained(a, references, seen) * evalChained(b, references, seen)
      case Divide(a, b) => evalChained(a, references, seen) / evalChained(b, references, seen)
    }
  }