User blog comment:Alemagno12/The Addition and Substraction functions./@comment-27173506-20151230053832

By the way, the X function does not support "-" or ":". The best you can do is S(Sn)=n.

Furthermore, your definition for A(n) is useless. Sn is already defined in the function and takes 2 symbols.

I wonder how will this develop?