apply-on-rectangle is a compiled Lisp function in `rect.el'.

(apply-on-rectangle FUNCTION START END &rest ARGS)

Call FUNCTION for each line of rectangle with corners at START, END.
FUNCTION is called with two arguments: the start and end columns of the
rectangle, plus ARGS extra arguments. Point is at the beginning of line when
the function is called.
The final point after the last operation will be returned.