Hi John, First of all: I'd like to ensure you had my response of may 27, since the mail system sent me today a notification saying it couldn't be delivered...?? When I search on Gmane I can see it so I think it's OK though... Second: in my previous mails I used the term "policy" where I should have used "error" since policies are rather the actions taken in case of error. Sorry for the terminological confusion... So finally, I think the best way is to create a new error. There are 2 options: either we call it zero_power_error and it's bound to be only used in pow, or we call it undetermined_result_error so that it can be eventually reused later for something else. The default policy for this error will be ignore_error. If we choose "zero_power_error", the result in case of ignore_error will be 1. If we choose "undetermined_result_error", the best way is to make the result depend on the function where it's used. In this case, the raise_undetermined_result_error will have to take that value in parameter. For example, in pow: return policies::raise_undetermined_result_error<T>( "boost::math::pow(%1%)", "The result of pow<0>(%1%) is undetermined", base, T(1), // value to return in case of ignore_error policy ); Which of the 2 options do you prefer? For the throw_on_error policy, do you see any existing exception (std or boost) that could fit or is it better to create a new one? For the errno_on_error policy, how about setting errno to EDOM and returning a NaN? Bruno