Why is the calculation formula of si (Shape index) inconsistent with the reference paper?

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Hi, In your paper and script source/masif_modules/read_data_from_surface.py, the calculation formula for si is "si = (k1+k2)/(k1-k2)" (k1>=k2), but the reference paper https://doi.org/10.1016/0262-8856(92)90076-F has the same calculation formula as "si = (k1+k2)/(k2-k1)" (k1>=k2). Your paper and the reference paper have the same explanation for the meaning of si. Why is that?