ListZipper.swift

  /// Insert a new track
  mutating func insert(element: T) {
    if let c = cursor {leftList.append(c)}
    
    cursor = element
  }

/// Remove the current pointed track
  mutating func remove() {
    guard let _ = cursor else {return}
    
    if leftList.isNotEmpty {
      cursor = leftList.popLast()
    } else {
      cursor = rightList.popLast()
    }
  }