functools.reduce accumulation

Question

Accepted Answer

`reduce` starts with the first element as the initial accumulator. The steps are: `(2 - 3) = -1`, then `(-1 - 4) = -5`, then `(-5 - 5) = -10`. The key insight is that `reduce` is left-associative - it processes elements left to right, folding each into the running result. Subtraction is not commutative or associative, so the order matters here. If you expected addition you'd get 14, but with subtraction the accumulation goes negative quickly.