  We provide a self-contained treatment of set-theoretic subsolutions to flow
by mean curvature, or, more generally, to flow by mean curvature plus an
ambient vector field. The ambient space can be any smooth Riemannian manifold.
Most importantly, we show that if two such set-theoretic subsolutions are
initially disjoint, then they remain disjoint as long as one of the
subsolutions is compact; previously, this was only known for Euclidean space
(with no ambient vectorfield). The new version (Sept 15, 2023) corrects a minor
typo.
