def mobile: Parser[Any] = ("[" ~> leftElements ~ "|" ~ rightElements <~ "]") ^^ { case left ~ "|" ~ right => Mobile(left ::: right, 0)}

def leftElements: Parser[List[Node]] = repsep(element, ",") ^^ {
  case nodes: List[Node] =>
    var pos = 1D
    nodes.reverse foreach {
      case s: Spacer =>
        pos += s.distance
      case n =>
        n.distance = pos
        pos += 1D
    }
    nodes
}

def rightElements: Parser[List[Node]] = repsep(element, ",") ^^ {
  case nodes: List[Node] =>
    var pos = 1D
    nodes foreach {
      case s: Spacer =>
        pos += s.distance
        s.distance = -s.distance
      case n =>
        n.distance = -pos
        pos += 1D
    }
    nodes
}

def element: Parser[Any] = number | spacer | mobile

def number: Parser[Leaf] = floatingPointNumber ^^ { case n => Leaf(n.toDouble, 0)}

def spacer: Parser[Spacer] = ("s" ~ floatingPointNumber) ^^ { case "s" ~ n => Spacer(n.toDouble)}