def new_bg(self,bg):
        """
        m.new_bg(,bg)  -- Change the ACGT background frequencies to those in the supplied dictionary.
                          Recompute log-likelihood, etc. with new background.
        """
        counts = []
        for pos in self.logP:
            D = {}
            for L,lp in pos.items():
                D[L] = math.pow(2.0,lp)
            counts.append(D)
        self.background = bg
        self.compute_from_counts(counts,0)
m.new_bg(,bg)  -- Change the ACGT background frequencies to those in the supplied dictionary.
                          Recompute log-likelihood, etc. with new background.