BoundaryMesh creates cells with inward-, not outward-pointing normal

mesh
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This documentation claims that

The cells of the boundary mesh … are oriented to produce outward pointing normals …

but according to my experience they produce inward pointing normals. Here is a minimal example:

from dolfin import *
import matplotlib.pyplot as plt
import numpy as np

N = 5
mesh = UnitSquareMesh(N,N)
bm = BoundaryMesh(mesh,"exterior",False)

fig,ax = plt.subplots()

for i in range(bm.num_cells()):
    cell = Cell(bm,i)
    n = cell.cell_normal()
    coords = cell.get_vertex_coordinates()
    x,y = coords[::2], coords[1::2]
    ax.plot(x,y,'k')
    ax.quiver(np.mean(x),np.mean(y),n.x(),n.y(),scale=10)

plt.show()

normals

I admit this is no big deal, but perhaps it might be of interest for some user.