Nostrum.Cache.GuildCache.select_by-exclamation-mark

You're seeing just the function select_by-exclamation-mark, go back to Nostrum.Cache.GuildCache module for more information.
Link to this function

select_by!(clauses, selector)

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Specs

select_by!(clauses(), selector()) :: any() | no_return()

Same as select_by/2, but raises Nostrum.Error.CacheError in case of failure.