  In the framework of Sobolev (Bessel potential) spaces $H^n(\reali^d, \reali
{or} \complessi)$, we consider the nonlinear Nemytskij operator sending a
function $x \in \reali^d \mapsto f(x)$ into a composite function $x \in
\reali^d \mapsto G(f(x), x)$. Assuming sufficient smoothness for $G$, we give a
"tame" bound on the $H^n$ norm of this composite function in terms of a linear
function of the $H^n$ norm of $f$, with a coefficient depending on $G$ and on
the $H^a$ norm of $f$, for all integers $n, a, d$ with $a > d/2$. In comparison
with previous results on this subject, our bound is fully explicit, allowing to
estimate quantitatively the $H^n$ norm of the function $x \mapsto G(f(x),x)$.
When applied to the case $G(f(x), x) = f^2(x)$, this bound agrees with a
previous result of ours on the pointwise product of functions in Sobolev
spaces.
