such that ``P`` is strictly convex, ``S`` is a subspace, and ``K``
is the direct sum of ``P`` and ``S``.
+ .. NOTE::
+
+ The name "Motzkin decomposition" is not standard. The result
+ is usually stated as the "decomposition theorem", or "cone
+ decomposition theorem."
+
OUTPUT:
An ordered pair ``(P,S)`` of closed convex polyhedral cones where