The series F(x) = 1 + x + x*x + ... converges only for |x| < 1. When we write F(F(x)), we write F(1 + x + x*x + ...). So, the argument of F is 1 + x + x*x + ..., which is not less than 1 if x > 0. So, we cannot interpret F(F(x)) as a power series in x. Similarly for F(F(F(x))). It is not justified to write out a power series in x for this function and to substitute. So, the counterpart of F(F(F(x))) in the type system is not List[List[List[X]]]. It is just X.