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Quite a lot of Emacs commands react to the (numeric) prefix. For
instance, C-u 4 C-f moves point four characters forward, and
C-u 9 0 0 I s s p adds a permanent
Subject substring score
rule of 900 to the current article.
This is all nice and well, but what if you want to give a command some additional information? Well, what most commands do is interpret the "raw" prefix in some special way. C-u 0 C-x C-s means that one doesn't want a backup file to be created when saving the current buffer, for instance. But what if you want to save without making a backup file, and you want Emacs to flash lights and play a nice tune at the same time? You can't, and you're probably perfectly happy that way.
I'm not, so I've added a second prefix--the symbolic prefix. The
prefix key is M-i (
gnus-symbolic-argument), and the next
character typed in is the value. You can stack as many M-i
prefixes as you want. M-i a C-M-u means "feed the C-M-u
command the symbolic prefix
a". M-i a M-i b C-M-u means
"feed the C-M-u command the symbolic prefixes
b". You get the drift.
Typing in symbolic prefixes to commands that don't accept them doesn't hurt, but it doesn't do any good either. Currently not many Gnus functions make use of the symbolic prefix.
If you're interested in how Gnus implements this, see section 10.7.7 Extended Interactive.