calculation of numerical derivatives fails in deltavar

This is a minor issue, but was vexing to track down so I thought I'd post this for posterity. For some users (apparently with a non-English installation of R?) the deltavar function can fail on line 36 because the error message returned by D ends up being in, for example, French - so rather than recognizing "is not in the derivatives table" the function fails to recognize "n'est pas dans la table des dérivées", and thus to calculate numerical derivatives. For the time being, I simply removed the if() statement checking the error message from my local invocation of the deltavar function.

Start at line 36 of the deltavar function and reproduce the failure under a non-English R installation. Check how the localized error from D is handled; done means numerical derivatives are calculated without relying on the English error text.

Written by the indexing model from the issue text.