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:     opK1K2 = optK1K2
    ELSE
      opK1K2 = 'l'
    ENDIF
  
    R = 83.14472_r8
  
negative.code:   !                **** BUT this should only be used when 2 < T < 35 and 19 < S < 43
    !       -> 'm10' means use Millero (2010) formulation for K1 & K2 (see Dickson et al., 2007)
    !                **** Valid for 0 < T < 50°C and 1 < S < 50 psu
    !     -----------
    !     optKf:
    !     ----------
    !       -> 'pf' means use Perez & Fraga (1987) formulation for Kf (recommended by Dickson et al., 2007)
    !               **** BUT Valid for  9 < T < 33°C and 10 < S < 40.
    !       -> 'dg' means use Dickson & Riley (1979) formulation for Kf (recommended by Dickson & Goyet, 1994)
    !     -----------
    !     optGAS: choose in situ vs. potential fCO2 and pCO2
    !     ---------
    !       PRESSURE corrections for K0 and the fugacity coefficient (Cf) 
    !       -> 'Pzero'   = 'zero order' fCO2 and pCO2 (typical approach, which is flawed)
    !                      considers in situ T & only atm pressure (hydrostatic=0)
    !       -> 'Ppot'    = 'potential' fCO2 and pCO2 (water parcel brought adiabatically to the surface)
    !                      considers potential T & only atm pressure (hydrostatic press = 0)
    !       -> 'Pinsitu' = 'in situ' fCO2 and pCO2 (accounts for huge effects of pressure)
    !                      considers in situ T & total pressure (atm + hydrostatic)
    !     ---------
  
    !     OUTPUT variables:
    !     =================
  