# V3 step 1.4 — module structure: three units + imports + opaque + nesting (done 2026-09-19)

Interactive grammar §2, landed as syntax-now/backend-later: everything
parses and type-checks, but only program modules lower to QBE.
Suite 9/9 (3 run + 6 reject), LL(1)-clean first try.

## Grammar (`compiler/src/M2.atg`)

- `M2 = Unit "."`, `Unit = DefUnit | ImplUnit | ProgModule`
  (FIRSTs `DEFINITION`/`IMPLEMENTATION`/`MODULE` — no conflicts).
- `DefUnit`: `DEFINITION MODULE` + imports + `CONST`/`TYPE`(opaque)/
  `VAR`, ends with one 230 (lowering = step 4). Procedure headings
  arrive with the declarations section (§next).
- `ImplUnit`: full `DeclSeq` + optional `BEGIN`, ends with one 230.
- `ProgModule`: as before + optional `Priority` (`"[" integer "]"`
  → 230) + `{ Import }`; only this path calls `QbeGen.EndModule`.
- `DeclSeq = { ConstBlock | VarBlock | NestedModule ";" }`.
- `NestedModule`: Wirth form — own scope via `PushScope`/`PopScope`,
  imports, `EXPORT [QUALIFIED] list ;` (parse-only, wires in step 4),
  optional `Priority`, decls, optional body **without** `BeginBody`
  (no fake `main` in the outer file), one 230 at `END`.
- `Import`: both `FROM A IMPORT x, y;` and `IMPORT A, B;` (V2 parity);
  names entered as `KindImport`, use-as-value already 230 via Design.
- `OpaqueBlock`: `TYPE T;` enters a targetless `NewAlias()` which
  resolves to `InvalidType` until step 4 completes it — cascades
  suppressed by the standard Invalid rules.

## Support changes

- `QbeGen.CloseModule` (def+mod): closes the `.ssa` with an honest
  `# no code` remark instead of a bogus `main`.
- 230 text reworded `"not supported in step 1 (integers only)"` →
  `"not supported yet"` (no tests referenced the old text).

## Tests

`t_defunit` (CONST+opaque+VAR definition → single 230, no cascades),
`t_implunit` (→ single 230), `t_nested` (inner `y := 2` checks clean
in its scope, `y` after `END Inner` → 201 proves the pop, + 230).

## Decisions locked (§2)

Three units now; both IMPORT forms; classic opaque `TYPE T;`;
nesting kept (Wirth form + EXPORT); priority accepted-but-230.
Next: § declarations (TYPE syntax, CONST expressions, procedure
headings — including `ProcHeading ";"` for DefUnit).