Thank you. I appreciated especially the pattern matching feature of C# 7 while programming the interpreter. For example:

        /// <summary>Evaluate a Lisp expression in an environment.</summary>
        public object Eval(object x, Cell env) {
            try {
                for (;;) {
                    switch (x) {
                    case Arg xarg:
                        return xarg.GetValue(env);
                    case Sym xsym:
                        try {
                            return Globals[xsym];
                        } catch (KeyNotFoundException) {
                            throw new EvalException("void variable", x);
                        }
                    case Cell xcell: