  We propose analogues of Green's and Picone's identities for the
$p$-sub-Laplacian on stratified Lie groups. In particular, these imply a
generalised D\'iaz-Sa\'a inequality for the $p$-sub-Laplacian on stratified Lie
groups. Using these we derive a comparison principle and uniqueness of a
positive solution to nonlinear hypoelliptic equations on general stratified Lie
groups extending to the setting of the general stratified Lie groups previously
known results on Euclidean and Heisenberg groups.
