single
Returns the tree's single leaf narrowed to its concrete node type N.
The load-boundary idiom: what an externally loaded asset tree contains is unknowable at compile time, so this places the one unavoidable runtime assertion at the load site — everything downstream works on the typed leaf cast-free, style and tags riding along.
Receiver
The single-leaf tree to narrow.
Return
The leaf as N.
Type Parameters
N
The expected concrete ShapeNode type.
Throws
when the tree holds no leaf at all.
when the tree holds more than one leaf.
when the single leaf is not an N — the message names the found and expected types.