anchor.diff:      type (heat_model), intent (inout) :: model
   
       model%t = 0.
  +    model%dt = 1.
       model%dx = 1.
       model%dy = 1.
   
  
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:      real :: dt
       real :: t
       real :: t_end
  
       real :: alpha
  
       integer :: n_x
       integer :: n_y
  