be sure that there's no name clash between NumericalDiff::df and the original functor df()
be sure that there's no name clash between NumericalDiff::df and the
original functor df()

rotorliu/eigen,madlib/eigen_backup,madlib/eigen_backup,mjbshaw/Eigen,mjbshaw/Eigen,madlib/eigen_backup,mjbshaw/Eigen,madlib/eigen_backup,rotorliu/eigen,rotorliu/eigen,rotorliu/eigen,mjbshaw/Eigen