public class Original
    {
        public string OldFunction(string s1, string s2)
        {
            return s1 + s2;
        }
    }


    public class RobertsSolution
    {
        // this potentially breaks old code unless new params are optional.
        // but raises concerns around ensuring validity of inputs.
        public string OldFunction(string s1, string s2, string newParam)
        {
            // this increases code complexity for all code paths
            // function gets longer and longer and longer and longer
            if (newParam != null)
            {
                s2 = newParam;
            }

            return s1 + s2;
        }
    }


    public class DansSolution
    {
        public string OldFunction(string s1, string s2)
        {
            return s1 + s2;
        }


        // this increases code complexity only for new code path
        // plus, the increase in complexity is smaller because there is no branching
        public string NewFunction(string newParam)
        {
            string s1 = null; // default value
            return OldFunction(s1, newParam);
        }

    }