anchor.diff:      dx2 = model%dx**2
       dy2 = model%dy**2
       dx2_dy2_rho = dx2 * dy2 * rho
  -    coef = model%dt / (2. * (dx2 + dy2))
  +    coef = model%alpha * model%dt / (2. * (dx2 + dy2))
   
       do i = 2, model%n_y-1
          do j = 2, model%n_x-1
  
positive.code:     type (heat_model), intent (inout) :: model
  
      model%t = 0.
      model%dx = 1.
      model%dy = 1.
  
  
negative.code:      real :: dt
       real :: t
       real :: t_end
  
       real :: alpha
  
       integer :: n_x
       integer :: n_y
  