  It is shown that grey soliton dynamics in an one-dimensional trap can be
treated as Landau dynamics of a quasi-particle. A soliton of arbitrary
amplitude moves in the trapping potential without deformation of its density
profile as a particle of mass $2m$. The dynamics in the local density
approximation is shown to be consistent with the perturbation theory for dark
solitons. Dynamics of a vortex ring in a trap is discussed qualitatively.
