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post.scala

val program2: (Position => Free[Instruction, Unit]) = {
    s: Position =>
      for {
        p1 <- forward(s, 10)
        p2 <- right(p1, Degree(90))
        p3 <- forward(p2, 10)
        p4 <- backward(p3, 20)//Here the computation stops, because result will be None
        _ <- showPosition(p4)
      } yield ()
  }