ListZipper.swift

  /// Select the left element if available
  mutating func left() {
    guard leftList.isNotEmpty else {return}
    
    if let c = cursor {rightList.append(c)}
    cursor = leftList.popLast()
  }
  
  /// Select the right element if available
  mutating func right() {
    guard rightList.isNotEmpty else {return}
    
    if let c = cursor {leftList.append(c)}
    cursor = rightList.popLast()
  }