  We consider the following polynomial generalization of Stern's diatomic
series: let $s_1(x,y)=1$, and for $n\geq 1$ set $s_{2n}(x,y)=s_n(x,y)$ and
$s_{2n+1}(x,y)=x\,s_n(x,y)+y\,s_{n+1}(x,y)$. The coefficient $[x^iy^j]s_n(x,y)$
is the number of hyperbinary expansions of $n-1$ with exactly $i$ occurrences
of the digit $\mathtt 2$ and $j$ occurrences of $\mathtt 0$. We prove that the
polynomials $s_n$ are invariant under \emph{digit reversal}, that is,
$s_n=s_{n^R}$, where $n^R$ is obtained from $n$ by reversing the binary
expansion of $n$.
