Corrigendum for

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H. Maarand, T. Uustalu. Reordering derivatives of trace closures of
regular languages.

In W. Fokkink, R. van Glabbeek, eds., CONCUR 2019, v. 140 of LIPIcs,
article 40. Dagstuhl Publishing, 2019. doi: 10.4230/lipics.concur.2019.40
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p 40:6, Lemma 15: Item 2 should read
"for every u, u' \in \Sigma^*, if \Sigma(u) = \Sigma(u'),
then R^I_u L = R^I_{u'} L."