Most design treats edge cases as exceptions to defend against — the rare input, the awkward state, the assumption that didn't hold. This surface takes the opposite stance: the edge is the frontier. The place a current pattern stops holding is exactly where the next generation of thinking has to start. edgecase is the working bench for that.
Where a pattern fails the unusual case, it's telling you what the next pattern must absorb. The failure mode is the spec for the upgrade.
Patterns don't mature in the happy path — they mature by accreting the cases they couldn't hold yesterday. Pattern-growth is an edge phenomenon.
Design from the hardest case inward and the common case comes for free. Design from the common case out and the edges stay broken forever.