Access the vertices associated to a facet

i would like to implement (in Python) a loop over all facets of my mesh, in which I mark all those facets, whose associated vertices satisfy a certain condition. 
Via facet.midpoint() I get the midpoint of a facet. 
But what about the "endpoints", i.e., the vertices?  How to access them? 

Related bugs

Related FAQ:

On Tue, Apr 09, 2013 at 12:06:08PM -0000, Thomas Betz wrote:
> New question #226265 on FEniCS Project:
> https://answers.launchpad.net/fenics/+question/226265
>
> Hi,
>
> i would like to implement (in Python) a loop over all facets of my mesh, in which I mark all those facets, whose associated vertices satisfy a certain condition. 
> Via facet.midpoint() I get the midpoint of a facet. 
> But what about the "endpoints", i.e., the vertices?  How to access them? 
>
> Thank you very much for your help.