rectangle-number-lines

rectangle-number-lines is an interactive autoloaded compiled Lisp function in `rect.el'.

It is bound to C-x r N.

(rectangle-number-lines START END START-AT &optional FORMAT)

Insert numbers in front of the region-rectangle.

START-AT, if non-nil, should be a number from which to begin
counting. FORMAT, if non-nil, should be a format string to pass
to `format' along with the line count. When called interactively
with a prefix argument, prompt for START-AT and FORMAT.