Unique Rectangle Burred Subset
Because the puzzle has exactly one solution, any pattern that would give two can be ruled out.
All candidates have been entered in the empty cells.
Assume that the puzzle has a unique solution. Use the equal-cell, equal-candidate set in A. Placing candidate 6 lets the two digits in A swap. Therefore, another solution exists. That contradicts uniqueness. Candidates 6 marked × can be removed.