 Controversially, this requires that we attach prior probabilities to the possible hypotheses as well as the parameters in our model. Pickett and Randle (2005) claim that it is impossible to assign priors in a consistent way. Velasco (2007) argues that this claim rests on a mistake, though the problem remains a difficult one. He later advocates assigning priors to tree topologies, but leaves open the more difficult problem of priors on branch lengths or model parameters (Velasco 2008b). Alfaro and Holder (2006) attempt to address some of these issues.