In page 46 in the programmar's
 

In page 46 in the programmar's guide, the governing equation for potentialFoam is written as div(U)==0 and Laplacian(p)==0, I know the first div(U)==0 can be easily obtained from the continuity equation, but I cannot derive this Laplacian(p)==0 from momentum equation, although it is evident that this relationship holds. How can I get this "Pressure equation"? Why the solver does not directly use the "momentum equation"?

I am a newbie in CFD, this might be a too simple question, but I am really confused, thanks a lot!