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Representing Key Sequences
A "complete key sequence" is a sequence of keystrokes that Emacs
understands as a unit. Key sequences are significant because you can
bind them to commands. Note that not all sequences of keystrokes are
possible key sequences. In particular, the initial keystrokes in a key
sequence must make up a "prefix key sequence".
Emacs represents a key sequence as a vector of keystrokes. Thus, the
schematic representation of a complete key sequence is as follows:
[(modifier .. modifer keysym) ... (modifier .. modifier keysym)]
Here are some examples of complete key sequences:
`[(control c) (control a)]'
Typing `C-c' followed by `C-a'
`[(control c) (control 65)]'
Typing `C-c' followed by `C-a'. (Using the ASCII code for the
`[(control c) (break)]'
Typing `C-c' followed by the `break' character.
A "prefix key sequence" is the beginning of a series of longer
sequences that are valid key sequences; adding any single keystroke to
the end of a prefix results in a valid key sequence. For example,
`control-x' is standardly defined as a prefix. Thus there is a
two-character key sequence starting with `C-x' for each valid
keystroke, giving numerous possibilities. Here are some samples:
* `[(control x) (c)]'
* `[(control x) (control c)]'
Adding one character to a prefix key does not have to form a complete
key. It could make another, longer prefix. For example, `[(control x)
(\4)]' is itself a prefix that leads to any number of different
three-character keys, including `[(control x) (\4) (f)]', `[(control x)
(\4) (b)]' and so on. It would be possible to define one of those
three-character sequences as a prefix, creating a series of
four-character keys, but we did not define any of them this way.
By contrast, the two-character sequence `[(control f) (control k)]'
is not a key, because the `(control f)' is a complete key sequence in
itself. You cannot give `[(control f (control k)]' an independent
meaning as a command while `(control f)' is a complete sequence,
because Emacs would understand C-f C-k as two commands.
The predefined prefix key sequences in Emacs are `(control c)',
`(control x)', `(control h)', `[(control x) (\4)]', and `escape'. You
can customize Emacs and could make new prefix keys or eliminate the
default key sequences. Note: Key Bindings. For example, if you
redefine `(control f)' as a prefix, `[(control f) (control k)]'
automatically becomes a valid key sequence (complete, unless you define
it as a prefix as well). Conversely, if you remove the prefix
definition of `[(control x) (\4)]', `[(control x) (\4) (f)]' (or
`[(control x) (\4) ANYTHING]') is no longer a valid key sequence.
Note that the above paragraphs uses \4 instead of simply 4, because
\4 is the symbol whose name is "4", and plain 4 is the integer 4, which
would have been interpreted as the ASCII value. Another way of
representing the symbol whose name is "4" is to write ?4, which would be
interpreted as the number 52, which is the ASCII code for the character
"4". We could therefore actually have written 52 directly, but that is
far less clear.
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