Can Adding a Constant to the D'Alembertian Maintain Relativistic Invariance?

[gulsen]

In Feynman lectures in physics v2 28-6, Feynman points out that we can add a constant times $\phi$ to D'Alembertian without distrupting the relativistic invariance. How and why?? Can someone work out a mathematical proof?

Thanks in advance.

 

[lalbatros]

The sum of two invariant quantities is an invariant quantity.

 

[Mentz114]

The derivative of a constant wrt any variable is zero.