/usr/lib/swipl/library/prolog_format.pl
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 format_spec(+Format, -Spec:list) is semidet
Parse a format string. Each element of Spec is one of the following:
text(Text)
Text sent to the output as is
escape(Num, Colon, Action)
A format escape. Num represents the optional numeric portion of an esape. Colon represents the optional colon in an escape. Action is an atom representing the action to be take by this escape.
ClioPatria (version V3.1.1-40-g9d9e003)