anchor.diff:    end function f_exp
   
   
  -  subroutine do_timestep_S (s, dt)
  +  subroutine do_timestep_S_imp (s, dt)
     !process the temporal evolution of S
       implicit none
   
  
positive.code:     if (info /= 0) then
        stop "error in DGTSV call in subroutine do_timestep_S"
      end if
    end subroutine do_timestep_S
  
    subroutine do_timestep_S_exp (states, dt)
      !process the temporal evolution of S with an explicit algorithm
  
negative.code:   implicit none
    private
  
    public :: do_timestep_T, do_timestep_S_imp, do_timestep_S_exp
  
  contains
    ! (T(t+dt) - T(t))/dt = f(s, dt)
  