added postfix syntax

docs/postfix-syntax.md

@@ -1 +1,87 @@
-# Postfix Syntax
+## Postfix Syntax
+
+Some languages are SOV — subject-object-verb — meaning the condition naturally comes before the keyword rather than after it. ForeignThon supports this with the `@@` operator.
+
+---
+
+## The problem
+
+In English-order Python, the keyword always comes first:
+
+```python
+if condition:
+    ...
+```
+
+In many languages, the natural order is the opposite — the condition is stated first, then the action. Forcing English word order on these languages makes the code feel unnatural.
+
+---
+
+## The solution
+
+The `@@` operator lets you put any keyword after its expression:
+
+```
+condition @@keyword:
+    body
+```
+
+This is equivalent to:
+
+```
+keyword condition:
+    body
+```
+
+Both produce identical compiled Python. `@@` is purely a source-level syntax — it is processed before tokenization and never appears in the output.
+
+---
+
+## Rules
+
+- `@@` rewrites only the line it appears on — nothing else changes
+- Indentation follows standard Python rules, unchanged
+- Prefix and postfix can be mixed freely in the same file
+- Works for any keyword in any language pack
+- `@@` is not valid Python syntax, so it never conflicts with existing code
+
+---
+
+## Supported constructs
+
+| Construct | Prefix | Postfix |
+|---|---|---|
+| if | `keyword condition:` | `condition @@keyword:` |
+| elif | `keyword condition:` | `condition @@keyword:` |
+| while | `keyword condition:` | `condition @@keyword:` |
+| def | `keyword name(args):` | `name(args) @@keyword:` |
+| class | `keyword Name:` | `Name @@keyword:` |
+| for | `keyword var in iter:` | `var @@in_kw iter @@for_kw:` |
+
+!!! note
+    `for` loops with postfix require two `@@` operators and can be complex. Most users keep `for` in prefix style.
+
+---
+
+## Decompile with postfix
+
+When converting Python back to a foreign language, pass `--postfix`:
+
+```bash
+fpy decompile script.py --lang <code> --postfix
+```
+
+Which keywords get rewritten is controlled by the `postfix_keywords` list in the language pack JSON. A language that uses SVO order sets this to `[]` — postfix output is never forced on languages that don't need it.
+
+```json
+"postfix_keywords": ["if", "elif", "while", "def", "class"]
+```
+
+---
+
+## Input vs output
+
+| Direction | Mechanism | Controlled by |
+|---|---|---|
+| Input (writing `.xx.py`) | `@@` in source | Always available for any keyword |
+| Output (`fpy decompile --postfix`) | Pack's `postfix_keywords` | Language pack author | Postfix Syntax