# Oberon-2 validator pilot

`oberon2_oberon.atg` describes Oberon-2 (Wirth/Mössenböck). It
generated cleanly with Coco/R V1.53 (`CR`) but needed three repairs
before GNU Modula-2 (`gm2 -fiso`) could build it and real programs
would parse — all diagnosed against `voc` as oracle:

1. Duplicate `CASE` labels in the generated `statement` procedure:
   ident-led `assignment` vs `ProcedureCall` merged into a single
   `IdentTail` dispatch (`":="` vs `"("`, plus selectors).
2. The classic optional-prefix trap in `qualident`: `[ident "."]`
   committed on any identifier and then demanded `.`, so every
   `TYPE T = INTEGER` failed (same fix as the Oberon-07 pilot:
   `ident { "." ident }`).
3. The reference grammar has no `FOR` statement at all — added
   `FOR ident := ... TO ... [BY ...] DO ... END` (verified against
   `voc`, which accepts `BY -1`).
4. The reference `string` token used `"` quotes; Oberon strings use
   `'` — fixed token and `noQuote` set.
5. The designator's type-guard alternative `"(" qualident ")"`
   shadowed calls with non-identifier arguments (`f(1)` failed while
   `f(i)` parsed as a guard); merged to `"(" [ ExpList ] ")"`.
6. Type-bound procedures gained their receiver:
   `PROCEDURE [(VAR) v: T] [Name*] ...` (verified against `voc`).

Two benign LL(1) warning families remain (dangling-`ELSE`-style,
loop-exit greediness — both resolve correctly).

Oberon-2 facts confirmed by probe (validator matches `voc`):
no enumerations, `WITH` only in guarded-pointer form,
`CASE` with `|` separators and `ELSE`, `LOOP`/`EXIT`, `RETURN`
with optional value, `&`/`~`/`OR`, `IN`/`IS`, sets with `..`
ranges, `NIL`, `NEW` (no dispose), `DIV`/`MOD`, `IMPORT` lists,
`FORWARD` (`PROCEDURE ^ name`) declarations.

Note: the reference `Grammars/Voc/voc_oberon.atg` is byte-identical
to the Oberon-2 grammar (modulo its header comment), and
`Grammars/Oberon-grammar/original_oberon.atg` differs only in its tags
line — this single validator covers all three.

## Build

```sh
./build.sh        # -> build/O2 (build/ is git-ignored)
```

Needs `CR` (Coco/R), `gm2 -fiso`, the `parser.frm`/`scanner.frm`
frames and `FileIO.def/.mod` — all resolved from the environment with
sensible defaults (`CRFRAMES`, `M2LIB`, `CR`, `GM2`).

## Differential results (O2 vs `voc`)

Accept battery (22 cases: all statements, `FOR`/`BY`, `CASE`/`ELSE`,
`LOOP`/`EXIT`, guarded `WITH`, `NEW`/deref, `ORD`, floats, strings,
`Out` calls, `RETURN`) + `tests/deep.Mod` (records, pointers,
multi-dim arrays, nested routines, type-bound procedures, all
statements): **0 false positives**. Broken mutants (9): **0 missed**,
positions close. (`i := 1` without `;` is accepted by `voc` too —
separator leniency, correctly skipped.)