 Toric contraction of a minus-one curve

A compact invariant curve with consecutive rays $u,v,w$ on a smooth toric surface contracts torically to a smooth point exactly when $u+w=v$, equivalently when its self-intersection is $-1$. Deleting $v$ then reverses a toric blowup at a torus-fixed point.

// Target: geometry-and-topology.bigb