code: def update(self):
          
          control = self.subpars.pars.control
          self(control.ft*control.fhru)
docs: Update |AbsFHRU| based on |FT| and |FHRU|.
  
          >>> from hydpy.models.lland import *
          >>> parameterstep('1d')
          >>> nhru(2)
          >>> lnk(ACKER)
          >>> ft(100.0)
          >>> fhru(0.2, 0.8)
          >>> derived.absfhru.update()
          >>> derived.absfhru
          absfhru(20.0, 80.0)