operate-on-rectangle

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

(operate-on-rectangle FUNCTION START END COERCE-TABS)

Call FUNCTION for each line of rectangle with corners at START, END.
If COERCE-TABS is non-nil, convert multi-column characters
that span the starting or ending columns on any line
to multiple spaces before calling FUNCTION.
FUNCTION is called with three arguments:
position of start of segment of this line within the rectangle,
number of columns that belong to rectangle but are before that position,
number of columns that belong to rectangle but are after point.
Point is at the end of the segment of this line within the rectangle.