What is the function of this prolog program function?

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I want to know what the function of function an is, thank you

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/ / Please paste the code text below (do not replace the code with pictures)
a ([], []).
a ([x | y], z):-a (y, w), append (w, [x], z).-sharp-sharp-sharp topic description

what result do you expect? What is the error message actually seen?

Mar.10,2022
MySQL Query : SELECT * FROM `codeshelper`.`v9_news` WHERE status=99 AND catid='6' ORDER BY rand() LIMIT 5
MySQL Error : Disk full (/tmp/#sql-temptable-64f5-1b3b3fb-2c26f.MAI); waiting for someone to free some space... (errno: 28 "No space left on device")
MySQL Errno : 1021
Message : Disk full (/tmp/#sql-temptable-64f5-1b3b3fb-2c26f.MAI); waiting for someone to free some space... (errno: 28 "No space left on device")
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