'StructureQuadMesh' object has no attribute 'cell_area'

[yoczhang]

from fealpy.mesh import StructureQuadMesh

box = [0, 1, 0, 1]
nx = 2
ny = 2
mesh = StructureQuadMesh(box, nx, ny)
cellarea = mesh.entity_measure('cell')

then, the error is

Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/Users/FEALPy/fealpy/fealpy/mesh/Mesh2d.py", line 79, in entity_measure
    return self.cell_area(index)
AttributeError: 'StructureQuadMesh' object has no attribute 'cell_area'