A type variable is marked, because absence is a bad thing to give meaning to

NEXT.md

@@ -1,3 +1,37 @@
+## Decided by the author, 2026-09-13: a type variable takes a `$` sigil
+
+**This overrides plan.org**, which says under Types: *"Lowercase type names are variables, Capitalized are
+concrete — no sigil."* That rule is withdrawn. Taken as provisional, to make progress rather than as a final
+design, and expected to be revisited after some real use.
+
+```lisp
+(defn swap [xs [$t] i i32 j i32] () ...)
+```
+
+Three reasons, the third being the one that actually forced it.
+
+1. **There is no binding site without a sigil.** Nothing distinguishes where a variable is introduced from where
+   it is used.
+2. **Introducing one is invisible.** `check.ml:535-562` resolves builtins, aliases, structs, unions, enums, then
+   near-miss, and *then* treats any leftover lowercase name as a type variable. So a lowercase name nobody has
+   heard of silently becomes a type parameter, and a mistyped type makes the function **more** permissive instead
+   of failing. The `near_miss` guard at line 553 exists for exactly this hazard and only reaches names close to a
+   known one — the comment there says so. `[elem]` is not close to anything and would become generic silently.
+3. **Brackets carry two opposite rules for the same lexical thing.** In `[n t]`, `check.ml:563` resolves `n`
+   against `env.consts` and *errors* if it is absent, while `t` becomes a *variable* if absent. Absence means
+   "mistake" on one side of the bracket and "new parameter" on the other.
+
+The rule is also not the one plan.org states. It is not case-based: `i32`, `f32`, `bool` and `string` are all
+lowercase and concrete. What the code actually does is **treat any unresolved lowercase name as a variable**,
+which is an absence rule, and absence is a bad thing to give meaning to.
+
+**Open, and deliberately not decided here:** whether a *use* after the binding is `$t` or bare `t`. Odin binds with
+`$T` and uses bare `T`. Left to whoever builds it, to be reported with the reason.
+
+**Also open: `$n` in length position**, Odin's `$N: int`, which is what would make a function generic over array
+length rather than only element type. Not in the spike unless it falls out for free; the spike is to report what it
+would cost.
+
 ## To discuss: five gaps the raylib examples hit and could not close
 
 Found by the lane that ported `core-2d-camera`, `core-scissor-test`, `core-window-flags`,