anchor.diff:       real :: dt
        real :: t
        real :: t_end
  +
  +     real :: alpha
   
        integer :: n_x
        integer :: n_y
  
positive.code:   subroutine initialize_from_defaults(model)
      type (heat_model), intent (out) :: model
  
      model%dt = 1.
      model%t_end = 20.
      model%n_x = 10
      model%n_y = 20
  
negative.code:     type (heat_model), intent (out) :: model
  
      open(15, file=config_file)
      read(15, *) model%alpha, model%t_end, model%n_x, model%n_y
      close(15)
      call initialize(model)
    end subroutine initialize_from_file
  