code: def update(self):
          
          con = self.subpars.pars.control
          self(con.tt+con.dttm)
docs: Update |TTM| based on :math:`TTM = TT+DTTM`.
  
          >>> from hydpy.models.hland import *
          >>> parameterstep('1d')
          >>> nmbzones(1)
          >>> zonetype(FIELD)
          >>> tt(1.0)
          >>> dttm(-2.0)
          >>> derived.ttm.update()
          >>> derived.ttm
          ttm(-1.0)