Add patch corresponding to `augref` to `pset`.

        `augref` must be either 'augment' or 'refine' statement.

def add_patch(self, pset, augref):
        """Add patch corresponding to `augref` to `pset`.

        `augref` must be either 'augment' or 'refine' statement.
        """
        try:
            path = [ self.add_prefix(c, augref)
                     for c in augref.arg.split("/") if c ]
        except KeyError:
            # augment of a module that's not among input modules
            return
        car = path[0]
        patch = Patch(path[1:], augref)
        if car in pset:
            sel = [ x for x in pset[car] if patch.path == x.path ]
            if sel:
                sel[0].combine(patch)
            else:
                pset[car].append(patch)
        else:
            pset[car] = [patch]