docs: def _convert(x, factor1, factor2):
          """
          Converts mixing ratio x in comp1 - comp2 tie line to that in
          c1 - c2 tie line.
  
          Args:
              x (float): Mixing ratio x in comp1 - comp2 tie line, a float
                  between 0 and 1.
              factor1 (float): Compositional ratio between composition c1 and
                  processed composition comp1. E.g., factor for
                  Composition('SiO2') and Composition('O') is 2.0.
              factor2 (float): Compositional ratio between composition c2 and
                  processed composition comp2.
  
          Returns:
              Mixing ratio in c1 - c2 tie line, a float between 0 and 1.
          """
          return x * factor2 / ((1-x) * factor1 + x * factor2)