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+# PIM4 vs GNU Modula-2 vs V3 — feature comparison
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+
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+**Date:** 2026-10-22 · **V3 state:** `v3-proc-types` (suite 255/255,
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+self-hosting fixpoint OK 3,935,235 bytes, corpus 503/612, 0 crashes).
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+
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+**Scope.** **PIM4** is Wirth's *Programming in Modula-2*, 4th edition
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+(the language specification, not a compiler). **GNU Modula-2 (gm2)** is
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+the GCC front end: its PIM and ISO modes plus the extensions common in
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+its testsuite. **V3** is this compiler: a PIM/ISO subset plus the gm2
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+compatibility it deliberately chose.
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+
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+**Legend:** ✅ supported / standard · 🔄 partial or caveated · ❌ absent ·
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+⚠️ non-standard extension · n/a not applicable.
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+
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+The tables describe the common/default behaviour; the exact flags and
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+modes of gm2 can change individual cells. V3's deliberate deviations
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+are listed at the end.
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+
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+## Standards & dialect
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+
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+| Feature | PIM4 | GNU Modula-2 | V3 |
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+|---|---|---|---|
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+| PIM base language | ✅ (definition) | ✅ (PIM 2/3/4 modes) | ✅ (PIM/ISO subset) |
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+| ISO 10514-1 base | ❌ (predates ISO) | ✅ | 🔄 mostly; see missing ISO rows below |
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+| ISO 10514-2 generics | ❌ | ✅ | ✅ (refinement form) |
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+| ISO 10514-3 OO | ❌ | ❌ | ❌ (`CLASS` is Clarion-form, not ISO OO) |
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+| Declaration before use | ✅ strict | 🔄 multi-pass, accepts forward refs | ✅ strict for data; 🔄 forward **procedure** calls allowed |
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+| `FORWARD` declarations | ✅ | ✅ | ✅ |
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+| `EXPORT QUALIFIED` / `UNQUALIFIED` | ❌ (PIM export list only) | ✅ | ✅ |
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+| Foreign modules (`DEFINITION MODULE FOR "C"`) | ❌ | ✅ | ✅ |
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+
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+## Lexical
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+
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+| Feature | PIM4 | GNU Modula-2 | V3 |
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+|---|---|---|---|
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+| Identifiers with `_` | ❌ (classic names) | ✅ | ✅ |
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+| Numeric bases | ✅ octal `B`, hex `H` | ✅ `B`/`H` (and more) | ❌ (`0x`/`0X` only; `012H`/`666B` rejected) |
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+| Nested `(* *)` comments | ✅ | ✅ | ✅ |
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+| `'…'` and `"…"` strings | ✅ | ✅ | ✅ |
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+| Unicode `UCHAR`/`UString` | ❌ | ❌ | ✅ (extension: `U'a'`, `U"…"`, `UCHR`, `UORD`) |
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+
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+## Types
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+
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+| Feature | PIM4 | GNU Modula-2 | V3 |
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+|---|---|---|---|
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+| `INTEGER`/`CARDINAL`/`SHORTINT`/`LONGINT`, `REAL`/`LONGREAL`, `BOOLEAN`, `CHAR` | ✅ | ✅ | ✅ |
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+| Subranges | ✅ | ✅ | ✅ (typed CHAR/enum subranges since 2026-10-22) |
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+| Enumerations | ✅ | ✅ | ✅ |
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+| Sets (`SET OF`) | ✅ any finite ordinal (impl. limit) | ✅ | 🔄 bool/char/enum/small-int subrange; span ≤ 65536; no `SET OF CARDINAL` |
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+| `PACKEDSET` | ❌ (ISO) | ✅ | ❌ |
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+| Fixed arrays, multi-dimensional | ✅ `ARRAY [a], [b] OF` | ✅ | ✅ both `[a], [b]` and `[a, b]` |
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+| Open arrays (1-D) | ✅ | ✅ | ✅ |
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+| Multi-dimensional open arrays | ❌ (ISO addition) | ✅ | ❌ (V3 descriptor-per-level representation; `unbounded*` gaps) |
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+| Records, variant records | ✅ | ✅ | ✅ (plus empty arms/`ELSE` extensions) |
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+| Pointers, `NIL`, opaque types | ✅ | ✅ | ✅ |
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+| Procedure types | ✅ (`PROCEDURE`) | ✅ (`PROCEDURE`, `PROC`) | ✅ (`PROCEDURE`, `PROC`; comma/unnamed forms) |
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+| Complex types (`COMPLEX`, `LONGCOMPLEX`) | ❌ | ✅ (ISO) | ❌ (`COMPLEX64` is a `REAL` placeholder, no arithmetic) |
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+| SYSTEM sized types (`WORD32`, `CARDINAL8`, `BITSET32`, …) | ❌ | ✅ | ✅ names, but mapped to V3's 32-bit/64-bit model |
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+| `BYTE` granularity | ✅ 8-bit | ✅ 8-bit | 🔄 `BYTE` = 32-bit `CARDINAL`; `ARRAY OF BYTE` is not byte-granular |
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+| `SHORTREAL` (32-bit) | ❌ | ✅ | ❌ (all reals are 8 bytes) |
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+| `ADDRESS` arithmetic (`ADDADR`/`SUBADR`/`DIFADR`) | ❌ | ✅ | ❌ |
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+| Absolute variables `VAR [addr]: T` | ❌ | ✅ (extension) | ❌ |
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+| Set ordering (`<`, `<=`, `>`, `>=`) | ❌ (`=`/`#`/`IN` only) | ✅ (extension) | ❌ |
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+
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+## Statements & procedures
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+
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+| Feature | PIM4 | GNU Modula-2 | V3 |
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+|---|---|---|---|
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+| `IF`/`CASE`/`WHILE`/`REPEAT`/`LOOP`/`EXIT`/`WITH`, `RETURN`, `HALT` | ✅ | ✅ | ✅ |
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+| `FOR` over any ordinal (CHAR/enum) | ✅ | ✅ | ✅ (since 2026-10-22) |
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+| `GOTO` / labels | ✅ | ✅ | ✅ |
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+| `INC`/`DEC` on enumerations | ❌ (integer) | ✅ | ❌ (integer only) |
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+| Empty string `""`/`''` as `CHAR` | ❌/unclear (ISO rule) | ✅ | ✅ (ISO rule, since 2026-10-22) |
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+| `LENGTH` | ❌ (ISO adds it) | ✅ | ✅ |
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+| `HIGH` | ✅ | ✅ | ✅ / 🔄 (`HIGH` of a string `CONST` still missing) |
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+| `TSIZE`/`SIZE`/`ADR` | ✅ | ✅ | ✅ |
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+| `SHIFT`/`ROTATE` | ⚠️ implementation SYSTEM extension | ✅ | ✅ |
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+| `TBITSIZE` | ❌ | ✅ | ❌ |
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+| ISO exceptions (`EXCEPT`, `RETRY`, `EXCEPTIONS`) | ❌ | ✅ | ❌ |
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+| Module finalization (`FINALLY`) | ❌ | ✅ | ❌ |
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+| `__FILE__` / `__LINE__` / `__FUNCTION__` | ❌ | ✅ | ✅ / ✅ / ❌ |
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+| `__BUILTIN__` / `__ATTRIBUTE__` pragmas | ❌ | ✅ | ✅ (parsed, ignored) |
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+| Inline `ASM` | ❌ | ✅ | ❌ |
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+
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+## Modules, compilation, libraries
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+
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+| Feature | PIM4 | GNU Modula-2 | V3 |
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+|---|---|---|---|
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+| Definition / implementation / program modules | ✅ | ✅ | ✅ |
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+| Local modules | ✅ | ✅ | ✅ |
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+| Separate compilation | ✅ | ✅ (object files/archives) | 🔄 one invocation compiles all units into one image; no incremental objects |
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+| Generics | ❌ | ✅ | ✅ |
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+| Object orientation | ❌ | ❌ | 🔄 Clarion-form `CLASS` (single inheritance, virtual dispatch), not ISO 10514-3 |
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+| PIM coroutines (`PROCESS`, `NEWPROCESS`, `TRANSFER`, `IOTRANSFER`) | ✅ | ✅ | ✅ (`IOTRANSFER` degrades to a transfer) |
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+| ISO `COROUTINES` module | ❌ | ✅ | ❌ (PIM interface only) |
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+| Standard library | ❌ (none defined) | ✅ PIM + ISO libraries | 🔄 subset: ISO/PIM I/O, strings, storage, SysClock; no ISO exceptions/termination libs |
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+| Self-hosting | n/a | ✅ (part of GCC) | ✅ (byte-identical 3-stage fixpoint) |
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+| Backend / target | n/a | GCC (many targets) | QBE → C (amd64 SysV today) |
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+
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+## The practical differences
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+
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+- **PIM4** is the clean specification: strict declaration order, no ISO
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+ exceptions/complex numbers/`PACKEDSET`/generics, no standard library,
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+ no sized SYSTEM types or address arithmetic.
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+- **gm2** is the superset in practice: PIM and ISO modes, generics,
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+ exceptions, complex numbers, `PACKEDSET`, multi-dimensional open
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+ arrays, sized SYSTEM types, address arithmetic, absolute variables,
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+ `ASM`, pragmas, and many extensions — but deliberately permissive
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+ (multi-pass resolution accepts forward references).
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+- **V3** is a PIM/ISO compiler with the opposite bias: it rejects gm2
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+ leniencies (declaration order), implements the common ISO/PIM
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+ constructs (generics, coroutines, foreign modules, ISO strings/SYSTEM/
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+ procedure types, empty string as `CHAR`), and documents its chosen gm2
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+ compatibility (`__FILE__`/`__LINE__`, scalar→`ARRAY OF BYTE`, same-size
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+ `VAR`, bare proc values, variant extensions, set `CASE` labels, libc
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+ prototypes). Its hard gaps are the large ISO features (**exceptions,
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+ complex arithmetic, `PACKEDSET`, finalization/protection,
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+ multi-dimensional open arrays, ISO OO**) and its **32-bit word model**
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+ (`BYTE` granularity, `SHORTREAL`, `REAL` width), plus a **65536-bit
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+ set span limit**.
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+
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+## V3-specific deviations and limits
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+
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+- **Strict data declaration order** (PIM-conforming); forward
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+ **procedure** calls are allowed where strict PIM would require
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+ `FORWARD`.
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+- **32-bit word model:** `INTEGER`/`CARDINAL`/`WORD`/`BYTE` are 32-bit;
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+ `LONGINT`/`LONGCARD` are 64-bit; every `REAL`/`SHORTREAL` is 8 bytes;
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+ `COMPLEX*` is a placeholder. `ARRAY OF BYTE` is a compatibility
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+ descriptor, not an 8-bit buffer.
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+- **Sets:** bases limited to bool/char/enum/small subranges; total span
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+ ≤ 65536; no `SET OF CARDINAL`.
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+- **No runtime range checks** on subrange assignment (V3 is already
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+ lenient for the integer family).
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+- **Multi-dimensional open arrays** are not supported: V3 represents an
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+ array level as its own descriptor, whereas gm2 passes a flat block —
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+ accepting the corpus files would compile but miscompile.
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+- **Single-invocation compilation:** definition, implementation and
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+ program units are compiled together into one image; there is no
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+ incremental object/archive format.
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+- **OO** is the Clarion-form `CLASS` dialect (single inheritance,
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+ virtual dispatch), not ISO 10514-3.
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+
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+## Sources
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+
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+- V3: `docs/STATUS.md`, `docs/features.md`, `docs/gaps/README.md`,
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+ `docs/analysis_gaps.md`, `docs/summary_session-2026-10-22.md`; the
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+ gates in this document were run at `v3-proc-types`.
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+- gm2: the GCC Modula-2 testsuite and `gm2-libs`/`gm2-libs-iso`
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+ definition modules in the sibling `gcc-git` tree.
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+- PIM4: N. Wirth, *Programming in Modula-2*, 4th edition.
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