Zero

Typeclass: a type with an identity element.

Zero[A] is the minimal requirement for weighted graph traversals that need a seed cost but do not accumulate costs along a path — specifically Prim's MST algorithm. Prim needs identity to seed the frontier but never calls combine.

Monoid[A] extends Zero[A] by adding combine, making Zero the honest supertype for the two-context-bound pattern E: {Zero, Ordering}.

Any given Monoid[A] automatically satisfies Zero[A] through inheritance.