An extension of partial covering fraction absorption into a power-law distribution of covering fraction as a function of column density, built from the wabs code. See Done & Magdziarz 1998 (MNRAS 298, 737) for details.
par1 = minimum equivalent hydrogen column (in units of 10**22 atoms/cm**2)
par2 = maximum equivalent hydrogen column (in units of 10**22 atoms/cm**2)
par3 = power law index for covering fraction.