code: def create_stream(self, assay=None, mfr=0.0, P=1.0, T=25.0,
                        normalise=True):
          
  
          if assay is None:
              return MaterialStream(self, self.create_empty_assay(), P, T)
  
          if normalise:
              assay_total = self.get_assay_total(assay)
          else:
              assay_total = 1.0
  
          return MaterialStream(self, mfr * self.converted_assays[assay] /
                                assay_total, P, T, self._isCoal(assay),
                                self._get_HHV(assay))
docs: Create a MaterialStream based on the specified parameters.
  
          :param assay: Name of the assay to be used to create the stream.
          :param mfr: Stream mass flow rate. [kg/h]
          :param P: Stream pressure. [atm]
          :param T: Stream temperature. [°C]
          :param normalise: Indicates whether the assay must be normalised
          before creating the Stream.
  
          :returns: MaterialStream object.