The subspace test asks for the zero vector first, before checking closure. Why is that the right order to apply the checks in, rather than a matter of taste?

A. Because it is the cheapest check and it disqualifies outright.

Both checks are necessary and cost very different amounts: the zero test is one evaluation, closure is a general argument about every pair. So the ordering is economics, run the cheap disqualifier first, and y = x + 1 is dismissed in a second. The test is not logically independent either: scaling any element by 0 produces zero, which is what makes it a good filter.