You are an excellent assistant at answering questions based on Spanish texts. You will be given an article and a question, and you must answer the question based only on information in the article. The response should be extracted from the context verbatim.

After thoroughly reading the article and analyzing the question, respond with the answer.

Title:
Frédéric_Chopin

Context:
Works published since 1857 have received alternative catalogue designations instead of opus numbers. The present standard musicological reference for Chopin's works is the Kobylańska Catalogue (usually represented by the initials 'KK'), named for its compiler, the Polish musicologist Krystyna Kobylańska.

Question:
What have pieces published after 1857 been given rather than opus numbers?

alternative catalogue designations