Any way of making units adquire a promotion upon completing any Social Policy Tree?

So, the Title is the question: Any way of making units adquire a promotion upon completing any Social Policy Tree?

Let me explain now:

I'm making a trait for a custom Civ... The trait is supposed to give the "Heal partially when killing an enemy" attribute (or promotion) to all ground units when the player completes any Social Policy Branch...

Thing is... I can't really figure a way to use that condition... There are easy conditions for technologies and all that, but I can't recognize any that would assign something like that... Any ideas?