  This paper shows that---under suitable conditions on a cone $C$---any element
in the convex hull of a decomposably $C$-antichain-convex set $Y$ is $C$-Pareto
dominated by some element of $Y$. Building on this, the paper proves the
disjointness of the convex hulls of two disjoint decomposably
$C$-antichain-convex sets whenever one of latter is $C$-upward. These findings
are used to obtain several consequences on the structure of the $C$-Pareto
optima of decomposably $C$-antichain-convex sets, on the separation of
decomposably $C$-antichain-convex sets and on the convexity of the set of
maximals of $C$-antichain-convex relations and of the set of maximizers of
$C$-antichain-quasiconcave functions. Special emphasis is placed on the
invariance of the solution set of a problem after its "convexification".
