%
% add_external_reaction
%
%   add reactions that are purley external and do not involve cells
%
function add_external_reaction(reactants, products, rate)

% Variables
global external_reaction_reactants;
global external_reaction_products;
global external_reaction_rates;
global num_external_reactions;

num_external_reactions = num_external_reactions + 1;
disp(['External reaction ' num2str(num_external_reactions) ':' reactants '->' products]);

% populate reactant names and their indices
external_reaction_reactants(num_external_reactions,1) = {reactants};
reactant_indices = [];
for i=1:size(reactants,2)
    if (i==1)
        reactant_indices = [lookup_species(reactants(i))];
    else
        reactant_indices = [reactant_indices lookup_species(reactants(i))];
    end
end
external_reaction_reactants(num_external_reactions,2) = {reactant_indices};

% populate product names and their indices
external_reaction_products(num_external_reactions,1)  = {products};
product_indices = [];
for i=1:size(products,2)
    if (i==1)
        product_indices = [lookup_species(products(i))];
    else
        product_indices = [product_indices lookup_species(products(i))];
    end
end
external_reaction_products(num_external_reactions,2) = {product_indices};

% store reaction rate
external_reaction_rates(num_external_reactions,1)     = {rate};

end