Of course. Why is that remark relevant here? Mathematica does make its S-expressions available to the user at runtime. They are in fact much more available to t

Of course. Why is that remark relevant here? Mathematica does make its S-expressions available to the user at runtime. They are in fact much more available to the user at runtime than in Lisp because of Mathematica's term rewriting semantics. Furthermore, the transformation from M-expressions into S-expressions isn't a one-way street. By default the notebook interface will render an S-expression as its M-expression equivalent when such exists.