The first call to Lambda Function by std::accumulate uses the initial value 0 as 'a' and the first element of the vector as 'b'.

for these ex : std::vector{-1, 2, 3, 4, -5};

first imple : a = 0, b = -1 ==> resul a + std::max(0, b) = 0 + 0 = 0; then store the result to a.

Second imple : a = 0, b = 2 ==> resul a + std::max(0, b) = 0 + 2 = 2; then store the result to a.
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