Thread:Project:Support desk/Help with expr: function, please

I'm trying to get a #expr: function to work, but it doesn't, and instead I get the message. The expression I'm trying to evaluate is



where the substitutions have been underline before and after they have been made. Can someone please help me to understand what actually happens here, and how I should write this instead? The expression I really want to evaluate is of course not as simple as this one, but it included an #expr: function inside of a Simple recursion that was in turn inside of another #expr: function.