Hi! I am trying to model the electrodeposition of MnO2 in COMSOL. I have 3 equations for 3 charged species involved in the reaction as well as an equation for the potential. However this last equation is a constant multiple of the form of the first 3 equations, giving me 4 unknowns and only 3 linearly-independent equations. I do have an additional constraint, which solves for one species concentration, based on the other two. However, I do not know how to implement this in COMSOL. I tried using a "global expression" but I cannot figure out how to make this work. Any tips?