+ A strictly convex cone contains no lines, and thus no negative
+ multiples of any of its elements besides zero::
+
+ sage: set_random_seed()
+ sage: K = random_cone(max_ambient_dim=8, strictly_convex=True)
+ sage: x = random_element(K)
+ sage: x.is_zero() or not K.contains(-x)
+ True
+
+ The sum of random elements of a cone lies in the cone::
+
+ sage: set_random_seed()
+ sage: K = random_cone(max_ambient_dim=8)
+ sage: K.contains(sum([random_element(K) for i in range(10)]))
+ True
+