Word Learning - Track Vocabulary Progress with Spaced Repetition State

Question

## Problem

Design a vocabulary tracker. Words have a learning state: `NEW -> LEARNING -> REVIEW -> MASTERED`. Each time a user answers a word correctly, it advances one state. Each wrong answer resets it to `LEARNING`.

Implement:

```python
class VocabTracker:
    def __init__(self, words: list[str]): ...
    def answer(self, word: str, correct: bool) -> str:  # returns new state
    def due_words(self, state: str) -> list[str]:       # sorted alphabetically
    def mastery_rate(self) -> float:                    # fraction in MASTERED
```

## Example

```
vt = VocabTracker(["cat", "dog", "fox"])
vt.answer("cat", True)   -> "LEARNING"
vt.answer("cat", True)   -> "REVIEW"
vt.answer("cat", False)  -> "LEARNING"
vt.due_words("NEW")      -> ["dog", "fox"]
vt.mastery_rate()        -> 0.0
```

## Follow-ups

1. How would you persist state across sessions (serialization format)?
2. Add a `next_due_time` per word using an exponential backoff: LEARNING=1 day, REVIEW=4 days, MASTERED=14 days.
3. How would you extend this to support multiple users without duplicating the word list?
4. Design the database schema for this system.

Word Learning - Track Vocabulary Progress with Spaced Repetition State

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