answered 4 years ago
Define f(X*Y) as a function with 2 arguments, say f2(X,Y), and then compute f(X^7) recursively as f2(X,f2(X,f2(X,f2(X,f2(X,f2(X,f2(X,X)))))).
f(X*Y)
f2(X,Y)
f(X^7)
f2(X,f2(X,f2(X,f2(X,f2(X,f2(X,f2(X,X))))))