Function to call to get text cut from other programs.
Most window systems provide a facility for cutting and pasting
text between different programs, such as the clipboard on X and
MS-Windows, or the pasteboard on Nextstep/Mac OS.
This variable holds a function that Emacs calls to obtain text
that other programs have provided for pasting. The function is
called with no arguments. If no other program has provided text
to paste, the function should return nil (in which case the
caller, usually `current-kill', should use the top of the Emacs
kill ring). If another program has provided text to paste, the
function should return that text as a string (in which case the
caller should put this string in the kill ring as the latest
The function may also return a list of strings if the window
system supports multiple selections. The first string will be
used as the pasted text, but the other will be placed in the kill
ring for easy access via `yank-pop'.
Note that the function should return a string only if a program
other than Emacs has provided a string for pasting; if Emacs
provided the most recent string, the function should return nil.
If it is difficult to tell whether Emacs or some other program
provided the current string, it is probably good enough to return nil if the string is equal (according to `string=') to the last text Emacs provided.