code: def trim(self, lower=None, upper=None):
          
          if upper is None:
              upper = getattr(self.subpars.relwz, 'value', None)
          lland_parameters.ParameterSoil.trim(self, lower, upper)
docs: Trim upper values in accordance with :math:`RelWB \\leq RelWZ`.
  
          >>> from hydpy.models.lland import *
          >>> parameterstep('1d')
          >>> nhru(3)
          >>> lnk(ACKER)
          >>> relwz.values = 0.5
          >>> relwb(0.2, 0.5, 0.8)
          >>> relwb
          relwb(0.2, 0.5, 0.5)