Swift: How can a superlayer have no sublayers?

Question

How can a superlayer have no sublayers? Please see image above.

This is Xcode 6.3.

Answer 1

As discussed in the comments, one case where a layer could be a superlayer but not have sublayers is when it has a mask.

let layer = CALayer()
let mask = CALayer()
layer.mask = mask

Now mask.superlayer is layer, but layer.sublayers is nil.