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Solve the following problem using Python, implementing the functions described below, one line at a time
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Description:
 def computeNormals(self):
 """Compute cell and vertex normals for the actor's mesh. .. warning:: Mesh gets modified, can have a different nr. of vertices. """

        poly = self.polydata(False)
        pnormals = poly.GetPointData().GetNormals()
        cnormals = poly.GetCellData().GetNormals()
        if pnormals and cnormals:
            return self

        pdnorm = vtk.vtkPolyDataNormals()
        pdnorm.SetInputData(poly)
        pdnorm.ComputePointNormalsOn()
        pdnorm.ComputeCellNormalsOn()
        pdnorm.FlipNormalsOff()
        pdnorm.ConsistencyOn()
        pdnorm.Update()
        return self.updateMesh(pdnorm.GetOutput())