Unique Rectangle Type 1
Use solution uniqueness to remove a rectangle's two defining candidates from its only cell with extra candidates.
All candidates have been entered in the empty cells.
Assume that this puzzle has a unique solution. The 3 cells in A contain only 6 and 8. If any candidate marked × is true, all four cells in A and × contain only 6 and 8. Swapping 6 and 8 would then produce another solution. That contradicts uniqueness. Therefore, remove 6 and 8 marked ×.