Remove the sd-sub-borrow skill as it was redundant with the existing
"Two-digit with ones place borrowing" skill which naturally covers
problems like 52-17, 43-18, etc.
The skill was problematic because:
- digitRange: { min: 1, max: 1 } constrained both operands to single digits
- But the description "13-7, 15-8" implied 2-digit minus 1-digit
- This contradiction made it impossible to generate appropriate problems
- Students were seeing either 0% or 100% of the intended pattern
Rather than fix the complex asymmetric digit range logic, we're removing
the skill entirely. The progression now flows:
- sd-sub-no-borrow (single-digit without borrowing)
- td-sub-no-borrow (two-digit without borrowing)
- td-sub-ones-borrow (two-digit with ones place borrowing)
This provides a cleaner, more natural progression.
Changes:
- Remove sd-sub-borrow skill definition from skills.ts
- Remove 'sd-sub-borrow' from SkillId type union
- Update td-sub-no-borrow prerequisites to reference sd-sub-no-borrow
- Remove sd-sub-borrow from skillMigration.ts mapping
- Remove generateTeensMinusSingles() function from problemGenerator.ts
- Revert generateOnesOnlyBorrow() to standard logic
Also includes previous fixes:
- Fix AllSkillsModal tab button types to prevent modal closing
- Add operator-specific display rules for mixed mode
- Add borrowNotation and borrowingHints to displayRules schema
🤖 Generated with [Claude Code](https://claude.com/claude-code)
Co-Authored-By: Claude <noreply@anthropic.com>
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