In various models of matching markets, substitutable preferences constitute the largest
domain for which stable matchings are guaranteed to exist. Recently, Hatfield et al.(2012)
have proposed an efficient algorithm to test substitutability of strict preferences. In this note
we show how the algorithm by Hatfield et al. can be adapted in such a way that it can test
substitutability of weak preferences as well. When restricted to the domain of strict
preferences, our algorithm is faster than Hatfield et al.'s original algorithm by a linear factor.