**Problem:** Using Unicode characters '+' and '−' (U+2212) as operator flags
causes bugs when developers use the wrong character (ASCII '-' vs Unicode '−').
**Solution:** Changed operator discriminator values to alphanumeric strings:
- Addition: '+' → 'add'
- Subtraction: '−' → 'sub'
**Files changed:**
- types.ts: Updated AdditionProblem and SubtractionProblem operator types
- All operator assignments changed: operator: 'add' / operator: 'sub'
- All operator comparisons changed: === 'add' / === 'sub'
**Benefits:**
- No more Unicode character confusion
- Easier to type and read
- Less error-prone for future developers
- Consistent with WorksheetOperator type ('addition', 'subtraction', 'mixed')
🤖 Generated with [Claude Code](https://claude.com/claude-code)
Co-Authored-By: Claude <noreply@anthropic.com>
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