Anagrams.Bag.subtract C# (CSharp) Method

        public Bag subtract(Bag subtrahend)
        {
            string m = guts;
            string s = subtrahend.guts;
            string difference = "";

            while (true)
            {
                if (s.Length == 0) return new Bag(difference + m);
                if (m.Length == 0) return null;
                {
                    char s0 = s[0];
                    char m0 = m[0];
                    if (m0 > s0) return null;
                    if (m0 < s0)
                    {
                        m = m.Substring(1);
                        difference += m0;
                        continue;
                    }
                    System.Diagnostics.Trace.Assert(m0 == s0,
                        "internal error!  Aggggh");
                    m = m.Substring(1);
                    s = s.Substring(1);
                }
            }
        }

        static private string subtract_strings(string minuend, string subtrahend)
        {
            Bag m = new Bag(minuend);

            Bag s    = new Bag(subtrahend);
            Bag diff = m.subtract(s);

            if (diff == null)
            {
                return(null);
            }
            return(diff.AsString());
        }