  Based on a variant of the frequency function approach of Almgren, we
establish an optimal bound on the vanishing order of solutions to stationary
Schr\"odinger equations associated to a class of subelliptic equations with
variable coefficients whose model is the so-called Baouendi-Grushin operator.
Such bound provides a quantitative form of strong unique continuation that can
be thought of as an analogue of the recent results of Bakri and Zhu for the
standard Laplacian.
