// accumulate just the element of the range I've been asked to
void operator()(std::size_t begin, std::size_t end) {
value += std::accumulate(input.begin() + begin, input.begin() + end, 0.0);
}
// accumulate just the element of the range I've been asked to
void operator()(std::size_t begin, std::size_t end) {
value += std::accumulate(input.begin() + begin, input.begin() + end, 0.0);
}