I assume (but don't know) that this is an attempt to avoid returning `nil` for empty collections: `[].inject(:+)` is `nil`, but `[].sum` is `0`. Forcing the us

I assume (but don't know) that this is an attempt to avoid returning `nil` for empty collections: `[].inject(:+)` is `nil`, but `[].sum` is `0`. Forcing the use of the additive identity every time, rather than starting with the first element of the collection, is a good way of setting people up to get the correct behaviour on empty collections, even if they don't test for that case.