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To avoid confusion with options, if a filename begins with a hyphen, it must be disguised by a leading absolute or relative directory path, e.g. /tmp/-foo.bib or ./-foo.bib.
OPTvolume = "??",The OPT prefix ensures that the key is ignored by BibTeX, so that the question marks will not appear in an output .bbl file. The GNU Emacs bibtex-mode editing support has functions for removing the OPT prefixes, and so does bibclean(1).
The doubled question marks are distinguished from single ones that might legitimately appear in value strings, and also serve as a convenient regular-expression pattern for bibextract(1), allowing easy preparation of a printed listing of just those entries that have incomplete bibliographic data:
bibextract '' '[?][?]' BibTeXfiles | lpr
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