HashSet<string> reagentSet = new();

            List<Reagent> prevReagents = new();

        public List<(string, string)> MissingReactionPairs()

        {

            ReactionSet? reactions = ProfileManager.CurrentProfile?.Reactions;

            Debug.Assert(reactions != null);

            HashSet<string> reagentSet = new();

            List<Reagent> prevReagents = new();

            List<(string, string)> missingReactionPairs = new List<(string, string)>(16);

            foreach (ReagentQuantity ingredient in _recipe)

            {

                if (reagentSet.Count > 4) return missingReactionPairs; // no more reactions

                string reagentName = ingredient.Name;

                if (string.IsNullOrEmpty(reagentName)) continue;

                if (reagentSet.Contains(reagentName)) continue;

                reagentSet.Add(reagentName);

                Reagent reagent = ReagentService.GetReagent(reagentName);

                // Find reactions.

                foreach (Reagent prevReagent in prevReagents)

                {

                    Reaction? reaction = reactions.Find(prevReagent, reagent);

                    if (reaction == null)

                    {

                        missingReactionPairs.Add((prevReagent.Name, reagentName));

                    }

                }

                prevReagents.Add(reagent);

            }

            return missingReactionPairs;

        }