GNU gettext utilities: elisp-format
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15.3.9 Emacs Lisp Format Strings
Emacs Lisp format strings are documented in the Emacs Lisp reference, section Formatting Strings /@w , https://www.gnu.org/manual/elisp-manual-21-2.8/html_chapter/elisp_4.html#SEC75. Note that as of version 21, XEmacs supports numbered argument specifications in format strings while FSF Emacs doesn’t.
