matching

A warm-up that screens for the stack instinct. The signal is recognizing last-opened-first-closed maps to a stack, and handling the edge cases (leftover opens, early close). Here is the pattern.

Propensity scores promise to mimic an experiment from observational data. The signal is knowing what they can and cannot fix, the overlap trap, and why IPW blows up. Here is the answer that separates careful candidates.