Face iterator for polyhedra#

This iterator in principle works on every graded lattice, where every interval of length two has exactly 4 elements (diamond property).

It also works on unbounded polyhedra, as those satisfy the diamond property, except for intervals including the empty face. A (slightly generalized) description of the algorithm can be found in [KS2019].

Terminology in this module:

Coatoms – the faces from which all others are constructed in the face iterator. This will be facets or Vrep. In non-dual mode, faces are constructed as intersections of the facets. In dual mode, they are constructed theoretically as joins of vertices. The coatoms are repsented as incidences with the atoms they contain.
Atoms – facets or Vrep depending on application of algorithm. Atoms are represented as incidences of coatoms they are contained in.

EXAMPLES:

Construct a face iterator:

sage: P = polytopes.cuboctahedron()
sage: C = CombinatorialPolyhedron(P)
sage: it = C.face_generator()
sage: next(it)
A 0-dimensional face of a 3-dimensional combinatorial polyhedron

Construct faces by the dual or not:

sage: it = C.face_generator(algorithm='primal')
sage: next(it).dimension()
2

sage: it = C.face_generator(algorithm='dual')
sage: next(it).dimension()
0

For unbounded polyhedra only non-dual iteration is possible:

sage: P = Polyhedron(rays=[[0,0,1], [0,1,0], [1,0,0]])
sage: C = CombinatorialPolyhedron(P)
sage: it = C.face_generator()
sage: [face.ambient_Vrepresentation() for face in it]
[(A vertex at (0, 0, 0),
  A ray in the direction (0, 1, 0),
  A ray in the direction (1, 0, 0)),
 (A vertex at (0, 0, 0),
  A ray in the direction (0, 0, 1),
  A ray in the direction (1, 0, 0)),
 (A vertex at (0, 0, 0),
  A ray in the direction (0, 0, 1),
  A ray in the direction (0, 1, 0)),
 (A vertex at (0, 0, 0), A ray in the direction (1, 0, 0)),
 (A vertex at (0, 0, 0), A ray in the direction (0, 1, 0)),
 (A vertex at (0, 0, 0),),
 (A vertex at (0, 0, 0), A ray in the direction (0, 0, 1))]
sage: it = C.face_generator(algorithm='dual')
Traceback (most recent call last):
...
ValueError: dual algorithm only available for bounded polyhedra

Construct a face iterator only yielding dimension \(2\) faces:

sage: P = polytopes.permutahedron(5)
sage: C = CombinatorialPolyhedron(P)
sage: it = C.face_generator(dimension=2)
sage: counter = 0
sage: for _ in it: counter += 1
sage: print ('permutahedron(5) has', counter,
....:        'faces of dimension 2')
permutahedron(5) has 150 faces of dimension 2
sage: C.f_vector()
(1, 120, 240, 150, 30, 1)

In non-dual mode one can ignore all faces contained in the current face:

sage: P = polytopes.cube()
sage: C = CombinatorialPolyhedron(P)
sage: it = C.face_generator(algorithm='primal')
sage: face = next(it)
sage: face.ambient_H_indices()
(5,)
sage: it.ignore_subfaces()
sage: [face.ambient_H_indices() for face in it]
[(4,),
 (3,),
 (2,),
 (1,),
 (0,),
 (3, 4),
 (1, 4),
 (0, 4),
 (1, 3, 4),
 (0, 1, 4),
 (2, 3),
 (1, 3),
 (1, 2, 3),
 (1, 2),
 (0, 2),
 (0, 1, 2),
 (0, 1)]

sage: it = C.face_generator(algorithm='dual')
sage: next(it)
A 0-dimensional face of a 3-dimensional combinatorial polyhedron
sage: it.ignore_subfaces()
Traceback (most recent call last):
...
ValueError: only possible when not in dual mode

In dual mode one can ignore all faces that contain the current face:

sage: it = C.face_generator(algorithm='dual')
sage: next(it)
A 0-dimensional face of a 3-dimensional combinatorial polyhedron
sage: face = next(it)
sage: face.ambient_V_indices()
(6,)
sage: [face.ambient_V_indices() for face in it]
[(5,),
 (4,),
 (3,),
 (2,),
 (1,),
 (0,),
 (6, 7),
 (4, 7),
 (2, 7),
 (4, 5, 6, 7),
 (1, 2, 6, 7),
 (2, 3, 4, 7),
 (5, 6),
 (1, 6),
 (0, 1, 5, 6),
 (4, 5),
 (0, 5),
 (0, 3, 4, 5),
 (3, 4),
 (2, 3),
 (0, 3),
 (0, 1, 2, 3),
 (1, 2),
 (0, 1)]

sage: it = C.face_generator(algorithm='primal')
sage: next(it)
A 2-dimensional face of a 3-dimensional combinatorial polyhedron
sage: it.ignore_supfaces()
Traceback (most recent call last):
...
ValueError: only possible when in dual mode

ALGORITHM:

The algorithm to visit all proper faces exactly once is roughly equivalent to the following. A (slightly generalized) description of the algorithm can be found in [KS2019].

Initialization:

faces = [set(facet) for facet in P.facets()]
face_iterator(faces, [])

The function face_iterator is defined recursively. It visits all faces of the polyhedron \(P\), except those contained in any of visited_all. It assumes faces to be exactly those facets of \(P\) that are not contained in any of the visited_all. It assumes visited_all to be some list of faces of a polyhedron \(P_2\), which contains \(P\) as one of its faces:

def face_iterator(faces, visited_all):
    while facets:
        one_face = faces.pop()
        maybe_new_faces = [one_face.intersection(face) for face in faces]
...

At this point we claim that maybe_new_faces contains all facets of one_face, which we have not visited before.

Proof: Let \(F\) be a facet of one_face. We have a chain: \(P \supset{}\) one_face \({}\supset F\). By the diamond property, there exists a second_face with \(P \supset{}\) second_face \({}\supset F\).

Now either second_face is not an element of faces: Hence second_face is contained in one of visited_all. In particular, \(F\) is contained in visited_all.

Or second_face is an element of faces: Then, intersecting one_face with second_face gives F.

This concludes the proof.

Moreover, if an element in maybe_new_faces is inclusion-maximal and not contained in any of the visited_all, it is a facet of one_face. Any facet in maybe_new_faces of one_face is inclusion-maximal.

Hence, in the following loop, an element face1 in maybe_new_faces is a facet of one_face if and only if it is not contained in another facet:

...
        maybe_new_faces2 = []
        for i, face1 in enumerate(maybe_new_faces):
            if (all(not face1 < face2 for face2 in maybe_new_faces[:i])
                    and all(not face1 <= face2 for face2 in maybe_new_faces[i+1:])):
                maybe_new_faces2.append(face1)
...

Now maybe_new_faces2 contains only facets of one_face and some faces contained in any of visited_all. It also contains all the facets not contained in any of visited_all.

We construct new_faces as the list of all facets of one_face not contained in any of visited_all:

...
        new_faces = []
        for face1 in maybe_new_faces2:
            if all(not face1 < face2 for face2 in visited_all):
                new_faces.append(face1)
...

By induction we can apply the algorithm, to visit all faces of one_face not contained in visited_all:

...
        face_iterator(new_faces, visited_all)
...

Finally we visit one_face and add it to visited_all:

...
        visit(one_face)
        visited_all.append(one_face)

Note: At this point, we have visited exactly those faces, contained in any of the visited_all. The function ends here.

ALGORITHM for the special case that all intervals of the lattice not containing zero are boolean (e.g. when the polyhedron is simple):

We do not assume any other properties of our lattice in this case. Note that intervals of length 2 not containing zero, have exactly 2 elements now. But the atom-representation of faces might not be unique.

We do the following modifications:

To check whether an intersection of faces is zero, we check whether the atom-representation is zero. Although not unique, it works to distinct from zero.
The intersection of two (relative) facets has always codimension \(1\) unless empty.
To intersect we now additionally unite the coatom representation. This gives the correct representation of the new face unless the intersection is zero.
To mark a face as visited, we save its coatom representation.
To check whether we have seen a face already, we check containment of the coatom representation.

class sage.geometry.polyhedron.combinatorial_polyhedron.face_iterator.FaceIterator_base#

Bases: sage.structure.sage_object.SageObject

A base class to iterate over all faces of a polyhedron.

Construct all proper faces from the facets. In dual mode, construct all proper faces from the vertices. Dual will be faster for less vertices than facets.

See FaceIterator.

current()#

Retrieve the last value of next().

EXAMPLES:

sage: P = polytopes.octahedron()
sage: it = P.combinatorial_polyhedron().face_generator()
sage: next(it)
A 0-dimensional face of a 3-dimensional combinatorial polyhedron
sage: it.current()
A 0-dimensional face of a 3-dimensional combinatorial polyhedron
sage: next(it).ambient_V_indices() == it.current().ambient_V_indices()
True

dual#

ignore_subfaces()#

The iterator will not visit any faces of the current face.

Only possible when not in dual mode.

EXAMPLES:

sage: P = polytopes.Gosset_3_21()
sage: C = CombinatorialPolyhedron(P)
sage: it = C.face_generator(algorithm='primal')
sage: n_non_simplex_faces = 1
sage: for face in it:
....:     if face.n_ambient_Vrepresentation() > face.dimension() + 1:
....:         n_non_simplex_faces += 1
....:     else:
....:         it.ignore_subfaces()
....:
sage: n_non_simplex_faces
127

Face iterator must not be in dual mode:

sage: it = C.face_generator(algorithm='dual')
sage: _ = next(it)
sage: it.ignore_subfaces()
Traceback (most recent call last):
...
ValueError: only possible when not in dual mode

Ignoring the same face as was requested to visit only consumes the iterator:

sage: it = C.face_generator(algorithm='primal')
sage: _ = next(it)
sage: it.only_subfaces()
sage: it.ignore_subfaces()
sage: list(it)
[]

Face iterator must be set to a face first:

sage: it = C.face_generator(algorithm='primal')
sage: it.ignore_subfaces()
Traceback (most recent call last):
...
ValueError: iterator not set to a face yet

ignore_supfaces()#

The iterator will not visit any faces containing the current face.

Only possible when in dual mode.

EXAMPLES:

sage: P = polytopes.Gosset_3_21()
sage: C = CombinatorialPolyhedron(P)
sage: it = C.face_generator(algorithm='dual')
sage: n_faces_with_non_simplex_quotient = 1
sage: for face in it:
....:     n_facets = face.n_ambient_Hrepresentation(add_equations=False)
....:     if n_facets > C.dimension() - face.dimension() + 1:
....:         n_faces_with_non_simplex_quotient += 1
....:     else:
....:         it.ignore_supfaces()
....:
sage: n_faces_with_non_simplex_quotient
4845

Face iterator must be in dual mode:

sage: it = C.face_generator(algorithm='primal')
sage: _ = next(it)
sage: it.ignore_supfaces()
Traceback (most recent call last):
...
ValueError: only possible when in dual mode

join_of_Vrep(*indices)#

Construct the join of the Vrepresentatives indicated by the indices.

This is the smallest face containing all Vrepresentatives with the given indices.

The iterator must be reset if not newly initialized.

Note

In the case of unbounded polyhedra, the smallest face containing given Vrepresentatives may not be well defined.

EXAMPLES:

sage: P = polytopes.cube()
sage: it = P.face_generator()
sage: it.join_of_Vrep(1)
A 0-dimensional face of a Polyhedron in ZZ^3 defined as the convex hull of 1 vertex
sage: it.join_of_Vrep(1,2).ambient_V_indices()
(1, 2)
sage: it.join_of_Vrep(1,3).ambient_V_indices()
(0, 1, 2, 3)
sage: it.join_of_Vrep(1,5).ambient_V_indices()
(0, 1, 5, 6)

sage: P = polytopes.cross_polytope(4)
sage: it = P.face_generator()
sage: it.join_of_Vrep().ambient_V_indices()
()
sage: it.join_of_Vrep(1,3).ambient_V_indices()
(1, 3)
sage: it.join_of_Vrep(1,2).ambient_V_indices()
(1, 2)
sage: it.join_of_Vrep(1,6).ambient_V_indices()
(0, 1, 2, 3, 4, 5, 6, 7)
sage: it.join_of_Vrep(8)
Traceback (most recent call last):
...
IndexError: coatoms out of range

If the iterator has already been used, it must be reset before:

sage: P = polytopes.dodecahedron()                       # optional - sage.rings.number_field
sage: it = P.face_generator()                            # optional - sage.rings.number_field
sage: _ = next(it), next(it)                             # optional - sage.rings.number_field
sage: next(it).ambient_V_indices()                       # optional - sage.rings.number_field
(15, 16, 17, 18, 19)
sage: it.join_of_Vrep(1,10)                              # optional - sage.rings.number_field
Traceback (most recent call last):
...
ValueError: please reset the face iterator
sage: it.reset()                                         # optional - sage.rings.number_field
sage: it.join_of_Vrep(1,10).ambient_V_indices()          # optional - sage.rings.number_field
(1, 10)

In the case of an unbounded polyhedron, we try to make sense of the input:

sage: P = polytopes.cube()*Polyhedron(lines=[[1]])
sage: it = P.face_generator()
sage: it.join_of_Vrep(1)
A 1-dimensional face of a Polyhedron in ZZ^4 defined as the convex hull of 1 vertex and 1 line
sage: it.join_of_Vrep(0, 1)
A 1-dimensional face of a Polyhedron in ZZ^4 defined as the convex hull of 1 vertex and 1 line
sage: it.join_of_Vrep(0)
Traceback (most recent call last):
...
ValueError: the join is not well-defined

sage: P = Polyhedron(vertices=[[1,0], [0,1]], rays=[[1,1]])
sage: it = P.face_generator()
sage: it.join_of_Vrep(0)
A 0-dimensional face of a Polyhedron in QQ^2 defined as the convex hull of 1 vertex
sage: it.join_of_Vrep(1)
A 0-dimensional face of a Polyhedron in QQ^2 defined as the convex hull of 1 vertex
sage: it.join_of_Vrep(2)
Traceback (most recent call last):
...
ValueError: the join is not well-defined
sage: it.join_of_Vrep(0,2)
A 1-dimensional face of a Polyhedron in QQ^2 defined as the convex hull of 1 vertex and 1 ray

sage: P = Polyhedron(rays=[[1,0], [0,1]])
sage: it = P.face_generator()
sage: it.join_of_Vrep(0)
A 0-dimensional face of a Polyhedron in ZZ^2 defined as the convex hull of 1 vertex
sage: it.join_of_Vrep(1,2)
A 2-dimensional face of a Polyhedron in ZZ^2 defined as the convex hull of 1 vertex and 2 rays

meet_of_Hrep(*indices)#

Construct the meet of the facets indicated by the indices.

This is the largest face contained in all facets with the given indices.

The iterator must be reset if not newly initialized.

EXAMPLES:

sage: P = polytopes.cube()
sage: it = P.face_generator()
sage: it.meet_of_Hrep(1,2)
A 1-dimensional face of a Polyhedron in ZZ^3 defined as the convex hull of 2 vertices
sage: it.meet_of_Hrep(1,2).ambient_H_indices()
(1, 2)
sage: it.meet_of_Hrep(1,3).ambient_H_indices()
(1, 3)
sage: it.meet_of_Hrep(1,5).ambient_H_indices()
(0, 1, 2, 3, 4, 5)

sage: P = polytopes.cross_polytope(4)
sage: it = P.face_generator()
sage: it.meet_of_Hrep().ambient_H_indices()
()
sage: it.meet_of_Hrep(1,3).ambient_H_indices()
(1, 2, 3, 4)
sage: it.meet_of_Hrep(1,2).ambient_H_indices()
(1, 2)
sage: it.meet_of_Hrep(1,6).ambient_H_indices()
(1, 6)
sage: it.meet_of_Hrep(1,2,6).ambient_H_indices()
(1, 2, 6, 7)
sage: it.meet_of_Hrep(1,2,5,6).ambient_H_indices()
(0, 1, 2, 3, 4, 5, 6, 7)

sage: s = cones.schur(4)
sage: C = CombinatorialPolyhedron(s)
sage: it = C.face_generator()
sage: it.meet_of_Hrep(1,2).ambient_H_indices()
(1, 2)
sage: it.meet_of_Hrep(1,2,3).ambient_H_indices()
Traceback (most recent call last):
...
IndexError: coatoms out of range

If the iterator has already been used, it must be reset before:

sage: P = polytopes.dodecahedron()                       # optional - sage.rings.number_field
sage: it = P.face_generator()                            # optional - sage.rings.number_field
sage: _ = next(it), next(it)                             # optional - sage.rings.number_field
sage: next(it).ambient_V_indices()                       # optional - sage.rings.number_field
(15, 16, 17, 18, 19)
sage: it.meet_of_Hrep(9,11)                              # optional - sage.rings.number_field
Traceback (most recent call last):
...
ValueError: please reset the face iterator
sage: it.reset()                                         # optional - sage.rings.number_field
sage: it.meet_of_Hrep(9,11).ambient_H_indices()          # optional - sage.rings.number_field
(9, 11)

next()#: Must be implemented by a derived class.

only_subfaces()#

The iterator will visit all (remaining) subfaces of the current face and then terminate.

EXAMPLES:

sage: P = polytopes.cube()
sage: it = P.face_generator()
sage: next(it).ambient_H_indices()
()
sage: next(it).ambient_H_indices()
(0, 1, 2, 3, 4, 5)
sage: next(it).ambient_H_indices()
(5,)
sage: next(it).ambient_H_indices()
(4,)
sage: it.only_subfaces()
sage: list(f.ambient_H_indices() for f in it)
[(4, 5), (3, 4), (1, 4), (0, 4), (3, 4, 5), (0, 4, 5), (1, 3, 4), (0, 1, 4)]

sage: P = polytopes.Birkhoff_polytope(4)
sage: C = P.combinatorial_polyhedron()
sage: it = C.face_generator()
sage: next(it).ambient_H_indices(add_equations=False)
(15,)
sage: next(it).ambient_H_indices(add_equations=False)
(14,)
sage: it.only_subfaces()
sage: all(14 in f.ambient_H_indices() for f in it)
True

Face iterator needs to be set to a face first:

sage: it = C.face_generator()
sage: it.only_subfaces()
Traceback (most recent call last):
...
ValueError: iterator not set to a face yet

Face iterator must not be in dual mode:

sage: it = C.face_generator(algorithm='dual')
sage: _ = next(it)
sage: it.only_subfaces()
Traceback (most recent call last):
...
ValueError: only possible when not in dual mode

Cannot run only_subfaces after ignore_subfaces:

sage: it = C.face_generator()
sage: _ = next(it)
sage: it.ignore_subfaces()
sage: it.only_subfaces()
Traceback (most recent call last):
...
ValueError: cannot only visit subsets after ignoring a face

only_supfaces()#

The iterator will visit all (remaining) faces containing the current face and then terminate.

EXAMPLES:

sage: P = polytopes.cross_polytope(3)
sage: it = P.face_generator()
sage: next(it).ambient_V_indices()
(0, 1, 2, 3, 4, 5)
sage: next(it).ambient_V_indices()
()
sage: next(it).ambient_V_indices()
(5,)
sage: next(it).ambient_V_indices()
(4,)
sage: it.only_supfaces()
sage: list(f.ambient_V_indices() for f in it)
[(4, 5), (3, 4), (2, 4), (0, 4), (3, 4, 5), (2, 4, 5), (0, 3, 4), (0, 2, 4)]

sage: P = polytopes.Birkhoff_polytope(4)
sage: C = P.combinatorial_polyhedron()
sage: it = C.face_generator(algorithm='dual')
sage: next(it).ambient_V_indices()
(23,)
sage: next(it).ambient_V_indices()
(22,)
sage: it.only_supfaces()
sage: all(22 in f.ambient_V_indices() for f in it)
True

reset()#

Reset the iterator.

The iterator will start with the first face again.

EXAMPLES:

sage: P = polytopes.cube()
sage: C = P.combinatorial_polyhedron()
sage: it = C.face_generator()
sage: next(it).ambient_V_indices()
(0, 3, 4, 5)
sage: it.reset()
sage: next(it).ambient_V_indices()
(0, 3, 4, 5)

class sage.geometry.polyhedron.combinatorial_polyhedron.face_iterator.FaceIterator_geom#

Bases: sage.geometry.polyhedron.combinatorial_polyhedron.face_iterator.FaceIterator_base

A class to iterate over all geometric faces of a polyhedron.

Construct all faces from the facets. In dual mode, construct all faces from the vertices. Dual will be faster for less vertices than facets.

INPUT:

P – an instance of Polyhedron_base
dual – if True, then dual polyhedron is used for iteration (only possible for bounded Polyhedra)
output_dimension – if not None, then the FaceIterator will only yield faces of this dimension

EXAMPLES:

Construct a geometric face iterator:

sage: P = polytopes.cuboctahedron()
sage: it = P.face_generator()
sage: next(it)
A 3-dimensional face of a Polyhedron in ZZ^3 defined as the convex hull of 12 vertices

Construct faces by the dual or not:

sage: it = P.face_generator(algorithm='primal')
sage: _ = next(it), next(it)
sage: next(it).dim()
2

sage: it = P.face_generator(algorithm='dual')
sage: _ = next(it), next(it)
sage: next(it).dim()
0

For unbounded polyhedra only non-dual iteration is possible:

sage: P = Polyhedron(rays=[[0,0,1], [0,1,0], [1,0,0]])
sage: it = P.face_generator()
sage: [face.ambient_Vrepresentation() for face in it]
[(A vertex at (0, 0, 0),
  A ray in the direction (0, 0, 1),
  A ray in the direction (0, 1, 0),
  A ray in the direction (1, 0, 0)),
 (),
 (A vertex at (0, 0, 0),
  A ray in the direction (0, 1, 0),
  A ray in the direction (1, 0, 0)),
 (A vertex at (0, 0, 0),
  A ray in the direction (0, 0, 1),
  A ray in the direction (1, 0, 0)),
 (A vertex at (0, 0, 0),
  A ray in the direction (0, 0, 1),
  A ray in the direction (0, 1, 0)),
 (A vertex at (0, 0, 0), A ray in the direction (1, 0, 0)),
 (A vertex at (0, 0, 0), A ray in the direction (0, 1, 0)),
 (A vertex at (0, 0, 0),),
 (A vertex at (0, 0, 0), A ray in the direction (0, 0, 1))]
sage: it = P.face_generator(algorithm='dual')
Traceback (most recent call last):
...
ValueError: cannot iterate over dual of unbounded Polyedron

Construct a FaceIterator only yielding dimension \(2\) faces:

sage: P = polytopes.permutahedron(5)
sage: it = P.face_generator(face_dimension=2)
sage: counter = 0
sage: for _ in it: counter += 1
sage: print ('permutahedron(5) has', counter,
....:        'faces of dimension 2')
permutahedron(5) has 150 faces of dimension 2
sage: P.f_vector()
(1, 120, 240, 150, 30, 1)

In non-dual mode one can ignore all faces contained in the current face:

sage: P = polytopes.cube()
sage: it = P.face_generator(algorithm='primal')
sage: _ = next(it), next(it)
sage: face = next(it)
sage: face.ambient_H_indices()
(5,)
sage: it.ignore_subfaces()
sage: [face.ambient_H_indices() for face in it]
[(4,),
 (3,),
 (2,),
 (1,),
 (0,),
 (3, 4),
 (1, 4),
 (0, 4),
 (1, 3, 4),
 (0, 1, 4),
 (2, 3),
 (1, 3),
 (1, 2, 3),
 (1, 2),
 (0, 2),
 (0, 1, 2),
 (0, 1)]

sage: it = P.face_generator(algorithm='dual')
sage: _ = next(it), next(it)
sage: next(it)
A 0-dimensional face of a Polyhedron in ZZ^3 defined as the convex hull of 1 vertex
sage: it.ignore_subfaces()
Traceback (most recent call last):
...
ValueError: only possible when not in dual mode

In dual mode one can ignore all faces that contain the current face:

sage: P = polytopes.cube()
sage: it = P.face_generator(algorithm='dual')
sage: _ = next(it), next(it)
sage: next(it)
A 0-dimensional face of a Polyhedron in ZZ^3 defined as the convex hull of 1 vertex
sage: face = next(it)
sage: face.ambient_V_indices()
(6,)
sage: [face.ambient_V_indices() for face in it]
[(5,),
 (4,),
 (3,),
 (2,),
 (1,),
 (0,),
 (6, 7),
 (4, 7),
 (2, 7),
 (4, 5, 6, 7),
 (1, 2, 6, 7),
 (2, 3, 4, 7),
 (5, 6),
 (1, 6),
 (0, 1, 5, 6),
 (4, 5),
 (0, 5),
 (0, 3, 4, 5),
 (3, 4),
 (2, 3),
 (0, 3),
 (0, 1, 2, 3),
 (1, 2),
 (0, 1)]

sage: it = P.face_generator(algorithm='primal')
sage: _ = next(it), next(it)
sage: next(it)
A 2-dimensional face of a Polyhedron in ZZ^3 defined as the convex hull of 4 vertices
sage: it.ignore_supfaces()
Traceback (most recent call last):
...
ValueError: only possible when in dual mode