  The papers~\cite{hatfimmokomi11} and~\cite{azizbrilharr13} propose algorithms
for testing whether the choice function induced by a (strict) preference list
of length $N$ over a universe $U$ is substitutable. The running time of these
algorithms is $O(|U|^3\cdot N^3)$, respectively $O(|U|^2\cdot N^3)$. In this
note we present an algorithm with running time $O(|U|^2\cdot N^2)$. Note that
$N$ may be exponential in the size $|U|$ of the universe.
