If x does not equal y, it is still possible that z ⋅ x = z ⋅ y {\displaystyle z\cdot x=z\cdot y} , which would give Bob the wrong answer.
x と y が等しくない場合でも z ⋅ x = z ⋅ y {\displaystyle z\cdot x=z\cdot y} である可能性はあり、その場合ボブは答えを間違う。