anchor.diff:           - s%Cv * s%v / s%x * dT_over_dx) / s%Cv
     end function f
   
  -
     subroutine do_timestep_S (states)
     !process the temporal evolution of S
       use mod_variables
  
positive.code:   use mod_constants
    use mod_timestep
    use mod_read_parameters
  
    implicit none 
    private
  
negative.code: 
    subroutine do_timestep_T(states) 
    !process the temporal evolution of T
      implicit none
      
      type (state), intent(inout)                   :: states
  