Refactor `a_count_occurrences_by_grade()` #464

All 857 tests pass, 3 skipped in 1m 31s

1 files ± 0 84 suites ±0 1m 31s ⏱️ +18s
860 tests ± 0 857 ✅ + 8 3 💤 - 8 0 ❌ ±0
2 304 runs +457 2 277 ✅ +1 116 27 💤 - 659 0 ❌ ±0

Results for commit 9da3fff. ± Comparison against earlier commit e4a7829.