BitstylesPhoenix.Classnames.xclassnames

You're seeing just the macro xclassnames, go back to BitstylesPhoenix.Classnames module for more information.
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xclassnames(arg)

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Extended classnames.

e2e- classes are trimmed at compile time. This can be disabled for test environments via trim_e2e_classes.

Conveniently accepts a (whitespace-separated) string or a list. If the list contains any conditional expressions, they are evaluated at runtime.