  We extend Nguyen's characterization of Sobolev spaces $W^{1,p}$ to the
setting of PI-metric measure spaces such that at a.e. point the tangent space
(in the Gromov-Hausdorff sense) is unique and Euclidean with a fixed dimension.
We also generalize [CLL14] to PI-metric measure spaces such that at a.e. point
the tangent space is unique and equal to the Heisenberg group with a fixed
homogeneous dimension. The approach is easier and completely different from the
one in [Ngu06, Ngu08].
