code: def update(self):
          
          con = self.subpars.pars.control
          self(con.relwb*con.nfk)
docs: Update |WB| based on |RelWB| and |NFk|.
  
          >>> from hydpy.models.lland import *
          >>> parameterstep('1d')
          >>> nhru(2)
          >>> lnk(ACKER)
          >>> relwb(0.2)
          >>> nfk(100.0, 200.0)
          >>> derived.wb.update()
          >>> derived.wb
          wb(20.0, 40.0)