anchor.diff:    !       -> '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:
     !     =================
  
positive.code:   !> 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
  
  ! Output variables:
    !> air-to-sea CO2 flux <b>[mol/(m^2 * s)]</b>
  
negative.code:   CHARACTER(3) :: opB
    CHARACTER(2) :: opKf
    CHARACTER(3) :: opK1K2
    CHARACTER(7) :: opGAS
  
  ! Set defaults for optional arguments (in Fortran 90)
  ! Note:  Optional arguments with f2py (python) are set above with 
  