Checkpoint — Advanced Type System

The gate

Three functions, one graded unit, no scaffolding: reference identity that is null-safe, a classification table that orders its checks correctly, and a range function that treats absence as a first-class contract. Passing this checkpoint means the type system's semantics — not just its syntax — are yours.

If it fails: re-read which of the three behaviors each test isolates. The identity test distinguishes Equals from ReferenceEquals; the classification test is an ordering exercise; the range test is an absence-handling exercise.