+ Restrict this cone (up to linear isomorphism) to a vector subspace.
+
+ This operation not only restricts the cone to a subspace of its
+ ambient space, but also represents the rays of the cone in a new
+ (smaller) lattice corresponding to the subspace. The resulting cone
+ will be linearly isomorphic **but not equal** to the desired
+ restriction, since it has likely undergone a change of basis.
+
+ To explain the difficulty, consider the cone ``K = Cone([(1,1,1)])``
+ having a single ray. The span of ``K`` is a one-dimensional subspace
+ containing ``K``, yet we have no way to perform operations like
+ :meth:`dual` in the subspace. To represent ``K`` in the space
+ ``K.span()``, we must perform a change of basis and write its sole
+ ray as ``(1,0,0)``. Now the restricted ``Cone([(1,)])`` is linearly
+ isomorphic (but of course not equal) to ``K`` interpreted as living
+ in ``K.span()``.