A cone is said to be permutation-invariant if ``P(K)`` is a subset
of ``K`` for every permutation matrix ``P``.
- EXAMPLES:
+ SETUP::
sage: from mjo.cone.rearrangement import rearrangement_cone
+ sage: from mjo.cone.permutation_invariant import is_permutation_invariant
+
+ EXAMPLES::
+
sage: all([ is_permutation_invariant(rearrangement_cone(p,n))
....: for n in range(3, 6)
....: for p in range(1, n) ])