Lesson 3: Split the work into steps you can check

Module 5 · Lesson 3 · About 45 minutes · Needs: Module 5 Lesson 1 (the refund rule decide) and the guard ask from course-guide/02; Module 4 Lesson 2 helps (scoring against gold answers)

The situation

It is Wednesday 1 July 2026, week 5. Your refund prompt from Lesson 1 does everything in one go: it reads Ana Duarte's ticket T-1001, works out who she is, checks the invoice and decides. When it is wrong, nobody can say which part failed. Maya also wants a short digest of the morning queue.

Both problems have one fix: split the work into small steps, give each step one job, and check what passes between them. T-1001, a duplicate charge of 288 USD on invoice INV-2026-004512, is the running example.