code: def pacl_term(DiamTube, ConcClay, ConcAl, ConcNatOrgMat, NatOrgMat,
                 coag, material, RatioHeightDiameter):
      
      return (gamma_coag(ConcClay, ConcAl, coag, material, DiamTube,
                         RatioHeightDiameter)
              * (1 - gamma_humic_acid_to_coag(ConcAl, ConcNatOrgMat,
                                              NatOrgMat, coag))
              )
docs: Return the fraction of the surface area that is covered with coagulant
      that is not covered with humic acid.
  
      :param DiamTube: Diameter of the dosing tube
      :type Diamtube: float
      :param ConcClay: Concentration of clay in solution
      :type ConcClay: float
      :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
      :param material: Type of clay in suspension, e.g. floc_model.Clay
      :type material: floc_model.Material
      :param RatioHeightDiameter: Dimensionless ratio of clay height to clay diameter
      :type RatioHeightDiameter: float
  
      :return: fraction of the surface area that is covered with coagulant that is not covered with humic acid
      :rtype: float