'_add(a, b)' expands to 'a+b' which is trivial and not really necessary,
but is added for consistency's sake.
'_radd' was also missing and was added (again for consistency).
Also removed an inaccurate notice from the header.

This '_add(a, b)' expands to 'a+b' which is trivial and not really necessary, but is added for
consistency's sake.
Also '_radd' was missing, and was added (again for consistency).

This reverts commit 9e925431.

(Issue reported by Karsten Wiesner and Alex Nitz)