Empty LatticePolytope fails

[vbraun]

As return values for Polyhedron.lattice_polytope() we sometimes want to return empty lattice polytopes. Alas, this does not work:

Lsage: LatticePolytope( matrix(ZZ,3,0) )
---------------------------------------------------------------------------
IndexError                                Traceback (most recent call last)

/home/vbraun/opt/sage-4.5.3/devel/sage-main/<ipython console> in <module>()

/home/vbraun/Sage/sage/local/lib/python2.6/site-packages/sage/geometry/lattice_polytope.pyc in LatticePolytope(data, desc, compute_vertices, copy_vertices, n)
    264         return data
    265     else:
--> 266         return LatticePolytopeClass(data, desc, compute_vertices, copy_vertices, n)
    267 
    268 copy_reg.constructor(LatticePolytope)   # "safe for unpickling"

/home/vbraun/Sage/sage/local/lib/python2.6/site-packages/sage/geometry/lattice_polytope.pyc in __init__(self, data, desc, compute_vertices, copy_vertices, n)
    402             if compute_vertices:
    403                 self._vertices = data       # Temporary assignment
--> 404                 self._compute_dim(compute_vertices=True)
    405             else:
    406                 if copy_vertices:

/home/vbraun/Sage/sage/local/lib/python2.6/site-packages/sage/geometry/lattice_polytope.pyc in _compute_dim(self, compute_vertices)
    494         else:
    495             points = self._vertices.columns(copy=False)
--> 496         p0 = points[0]
    497         if p0 != 0:
    498             points = [point - p0 for point in points]

IndexError: list index out of range

The attached patch fixes this and adds a suitable doctest.

[vbraun]

Initial patch

[vbraun]

indentation fix in docstring

[novoselt]

Nice improvements!