code: def gamma_humic_acid_to_coag(ConcAl, ConcNatOrgMat, NatOrgMat, coag):
      
      return min(((ConcNatOrgMat / conc_precipitate(ConcAl, coag).magnitude)
                  * (coag.Density / NatOrgMat.Density)
                  * (coag.Diameter / (4 * NatOrgMat.Diameter))
                  ),
                 1)
docs: Return the fraction of the coagulant that is coated with humic acid.
  
      :param ConcAl: Concentration of alumninum in solution
      :type ConcAl: float
      :param ConcNatOrgMat: Concentration of natural organic matter in solution
      :type ConcNatOrgMat: float
      :param NatOrgMat: type of natural organic matter, e.g. floc_model.HumicAcid
      :type NatOrgMat: floc_model.Material
      :param coag: Type of coagulant in solution, e.g. floc_model.PACl
      :type coag: floc_model.Material
  
      :return: fraction of the coagulant that is coated with humic acid
      :rtype: float