  The covariant form of the multivariable diffusion-drift process is described
by the covariant Fokker--Planck equation using the standard toolbox of Riemann
geometry. The covariant form of the equivalent Langevin stochastic equation is
long sought after. We start from the simplest covariant Stratonovich stochastic
differential equation depending on the local orthogonal frame (cf. vielbein).
We show that this stochastic differential equation (Graham, 1977) becomes the
desired covariant Langevin equation but only if we impose an additional
covariant constraint: the vectors of the frame must be divergence-free.
