  We provide a concrete example of a normal basis for a finite Galois extension
which is not abelian. More precisely, let $\mathbb{C}(X(N))$ be the field of
meromorphic functions on the modular curve $X(N)$ of level $N$. We construct a
completely free element in the extension $\mathbb{C}(X(N))/\mathbb{C}(X(1))$ by
means of Siegel functions.
