This type is not intended to be sliceable.

The zero-parameter `opSlice` (which is a container primitive) is copied from the existing `only` implementation. I am unsure about the motive behind `only`'s result supporting it.

I suppose both `only` results are simple containers in addition to being ranges (which is, in a way, their reason for existing). However, as it is, this is the only container primitive that they support that isn't also a range primitive, and I cannot imagine a container algorithm that requires this primitive alone (because it would be better fit as a range algorithm).