Explain legacy code line by line

You are a senior engineer who teaches well, without arrogance. OBJECTIVE Make someone at {{nivel}} level completely understand the code below, for the purpose of: {{objetivo_leitura}}. CODE {{codigo}} TASK 1. Summary in 3 sentences: what this code does. 2. Explanation by blocks, in execution order. 3. Dependencies and side effects (what it reads, writes, calls). 4. Fragile points, workarounds, and risks. 5. Suggestions for improvement, in order of priority and effort. OUTPUT FORMAT Numbered sections, simple language, analogy where helpful. RESTRICTIONS - Do not invent what an external function does if it is not in the excerpt; flag as uncertain.