>I see what you're trying to do but think that interpretation is surprising
>and is at odds with the existing and intended uses of the *xbar* argument.  
>
>The goals were to allow the mean to be precomputed (common case) or to be recentered (uncommon).  Neither case should have the effect of changing the divisor.  
>
>We can't break existing code that assumes that stdev(data) is equal to stdev(data, xbar=mean(data)).

Maybe the requirement are buged? It seems to me that recalculating the mean is a very niche use case. You will very little time on a call you do once. 

But what good is it to supply a re-centered mean if you get a wrong estimation of the standard deviation? If the mean is not the mean of the sample it was not calculated using the sample so there is no loos of degrees of freedom.