anchor.diff:    !> for K1,K2 choose either \b 'l' (Lueker et al., 2000) or \b 'm10' (Millero, 2010) 
   !f2py character*3 optional, intent(in) :: optK1K2='l'
     CHARACTER(3), OPTIONAL, INTENT(in) :: optK1K2
  +  !> for K0,fugacity coefficient choose either \b 'Ppot' (no pressure correction) or \b 'Pinsitu' (with pressure correction) 
  +  !! 'Ppot'    - for 'potential' fCO2 and pCO2 (water parcel brought adiabatically to the surface)
  +  !! 'Pinsitu' - for 'in situ' values of fCO2 and pCO2, accounting for pressure on K0 and Cf
  +  !! with 'Pinsitu' the fCO2 and pCO2 will be many times higher in the deep ocean
  +!f2py character*7 optional, intent(in) :: optGAS='Pinsitu'
  +  CHARACTER(7), OPTIONAL, INTENT(in) :: optGAS
   
   ! Ouput variables
     !> solubility of CO2 in seawater (Weiss, 1974), also known as K0
  
positive.code:   CHARACTER(3) :: opB
    CHARACTER(2) :: opKf
    CHARACTER(3) :: opK1K2
  
    ! CONSTANTS
    ! =========
  
negative.code: !         Mehrbach et al. (1973) refit, by Lueker et al. (2000) (total pH scale)
            K1(i) = 10.0d0**(-1.0d0*(3633.86d0*invtk - 61.2172d0 + 9.6777d0*dlogtk  &
                    - 0.011555d0*s + 0.0001152d0*s2))
            K2(i) = 10.0d0**(-1*(471.78d0*invtk + 25.9290d0 - 3.16967d0*dlogtk      &
                    - 0.01781d0*s + 0.0001122d0*s2))
          ELSEIF (trim(opK1K2) == 'm10') THEN
  