code: def PorodGuinier(q, a, alpha, Rg):
      
      return PorodGuinierMulti(q, a, alpha, Rg)
docs: Empirical Porod-Guinier scattering
  
      Inputs:
      -------
          ``q``: independent variable
          ``a``: factor of the power-law branch
          ``alpha``: power-law exponent
          ``Rg``: radius of gyration
  
      Formula:
      --------
          ``G * exp(-q^2*Rg^2/3)`` if ``q>q_sep`` and ``a*q^alpha`` otherwise.
          ``q_sep`` and ``G`` are determined from conditions of smoothness at
          the cross-over.
  
      Literature:
      -----------
          B. Hammouda: A new Guinier-Porod model. J. Appl. Crystallogr. (2010) 43,
              716-719.