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The Body of copy-region-as-kill

The copy-region-as-kill function is written so that two or more kills in a row combine their text into a single entry. If you yank back the text from the kill ring, you get it all in one piece. Moreover, kills that kill forward from the current position of the cursor are added to the end of the previously copied text and commands that copy text backwards add it to the beginning of the previously copied text. This way, the words in the text stay in the proper order.

The function makes use of two variables that keep track of the current and previous Emacs command. The two variables are this-command and last-command.

Normally, whenever a function is executed, Emacs sets the value of this-command to the function being executed (which in this case would be copy-region-as-kill). At the same time, Emacs sets the value of last-command to the previous value of this-command. However, the copy-region-as-kill command is different; it sets the value of this-command to kill-region, which is the name of the function that calls copy-region-as-kill.

In the first part of the body of the copy-region-as-kill function, an if expression determines whether the value of last-command is kill-region. If so, the then-part of the if expression is evaluated; it uses the kill-append function to concatenate the text copied at this call to the function with the text already in the first element (the CAR) of the kill ring. On the other hand, if the value of last-command is not kill-region, then the copy-region-as-kill function attaches a new element to the kill ring.

The if expression reads as follows; it uses eq, which is a function we have not yet seen:

(if (eq last-command 'kill-region)
    ;; then-part
    (kill-append (buffer-substring beg end) (< end beg))

The eq function tests whether its first argument is the same Lisp object as its second argument. The eq function is similar to the equal function in that it is used to test for equality, but differs in that it determines whether two representations are actually the same object inside the computer, but with different names. equal determines whether the structure and contents of two expressions are the same.

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