They look like convenient additions... but I'd be reluctant to add so many category methods onto a Foundation class. Especially for un-prefixed methods.
If a future version of iOS adds an -sum method, for example, you'd better hope it has the exact same semantics as yours, otherwise if the system calls -sum expecting its implementation to be used, bad things (or worse, bad things with no apparent cause) will happen when your implementation is used instead.
If a future version of iOS adds an -sum method, for example, you'd better hope it has the exact same semantics as yours, otherwise if the system calls -sum expecting its implementation to be used, bad things (or worse, bad things with no apparent cause) will happen when your implementation is used instead.