foldl

This code calculates the sum of the elements in the collection, and then folds the result into the accumulator. The foldl function takes a function that takes two inputs: the current element and a default value. The function is called with the current element and the default value, and the result is returned to the foldl function. The Foldl function then uses the + operator to add the default value to the current element, and returns the result.