/usr/local/lib/swipl/library/prolog_source.pl
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 prolog:quasi_quotation_syntax(+Syntax, -Library) is semidet[multifile, library(prolog_source)]
True when the quasi quotation syntax Syntax can be loaded from Library. Library must be a valid first argument for use_module/2.

This multifile hook is used by library(prolog_source) to load quasi quotation handlers on demand.