This costed me 3 WAs during the contest!!!

What I did wrong

Given the sequence goes from beginning to end, in order, I should at least try validating in that order, i.e., classification based on the case if b1 can be written.

Instead, I classified on the value of q, and therefore missed one case where abs(b1) > l, i.e., I worried about the latter cases without thinking if the former, dependent cases are even valid to begin with.