code: def _g(self, h, xp, s):
          
  
          nphi = sum(self.phi)
  
          return (nphi / 2.0) * log(2 * pi) + nphi * \
              log(s) + 0.5 * sum((h - xp) ** 2) / (s ** 2)
docs: Density function for blow and hop moves