distinct type aliasing

TODO.md

@@ -127,7 +127,12 @@
 
 7. enums (native and c interop) (see below)
 
-8. distinct types (see below)
+8. distinct types (implemented; see below)
+  - nominal declarations preserve identity across packages and reuse the backing runtime representation
+  - construction uses `Type(value)` with exactly one value of the exact backing type
+  - no implicit conversion to or from the backing type
+  - backing-type operators and reverse explicit conversions remain deferred
+  - concrete runtime backing types are supported; unresolved, `int`, `void`, function, and opaque backings are rejected
 
 ## A word on multi-unwrap
 
@@ -204,7 +209,7 @@ Distinct types are considered distinct from their backing type. They do not impl
 UserID :: distinct u32
 
 # instantiate distinct type
-my_id UserId :: UserID(42) # value must be of to backing type
+my_id UserID :: UserID(42) # value must have the exact backing type
 ```
 
 # A word on enums