Returns an instance of
T calculated by summing all instances in iter in one pass.
Returns an instance of
T calculated by summing all instances in iter in one pass. Returns None if
iter is empty, else Some[
T].
None if iter is empty, else an option value containing the summed
T
Override if there is a faster way to compute this sum than iter.reduceLeftOption using plus.
Combine 9 semigroups into a product semigroup