code: def update(self):
          
          con = self.subpars.pars.control
          self(0.)
          for idx in range(2):
              if (con.bbv[idx] > 0.) and (con.bnv[idx] > 0.):
                  self.values[idx] = con.bbv[idx]/con.bnv[idx]
docs: Update value based on :math:`HV=BBV/BNV`.
  
          Required Parameters:
              |BBV|
              |BNV|
  
          Examples:
              >>> from hydpy.models.lstream import *
              >>> parameterstep('1d')
              >>> bbv(left=10., right=40.)
              >>> bnv(left=10., right=20.)
              >>> derived.hv.update()
              >>> derived.hv
              hv(left=1.0, right=2.0)
              >>> bbv(left=10., right=0.)
              >>> bnv(left=0., right=20.)
              >>> derived.hv.update()
              >>> derived.hv
              hv(0.0)