Comment by happy_dino

> I am still seeking the legendary JVM implementation with TCO.

  scala> :paste
  // Entering paste mode (ctrl-D to finish)

  object TCO extends App {
    def stackSize = Thread.currentThread.getStackTrace.length
    def printStackSizeMessageOutside() =
      println(s"$stackSize stackframes outside of the mutually recursive calls")
    def printStackSizeMessageInside() =
      println(s"$stackSize stackframes inside the mutually recursive calls")

    def mutuallyRecursiveAdd1(a: Long, b: Long): Long =
      if (b == 0) {
        printStackSizeMessageInside()
        a
      } else
        mutuallyRecursiveAdd2(a+1, b-1)
  
    def mutuallyRecursiveAdd2(a: Long, b: Long): Long =
      if (b == 0) {
        printStackSizeMessageInside()
        a
      } else
        mutuallyRecursiveAdd1(a+1, b-1)
  
    printStackSizeMessageOutside()
    mutuallyRecursiveAdd1(23, 10000000)
  }

  // Exiting paste mode, now interpreting.

  defined object TCO

  scala> TCO main null
  We have 29 stackframes outside of the mutually recursive calls
  We have 30 stackframes inside the mutually recursive calls