--- rect-orig.el	2023-01-31 17:25:08.498658466 +0300
+++ rect-patched.el	2023-01-31 17:30:02.163121185 +0300
@@ -144,11 +144,16 @@
 
 ;;; Rectangle operations.
 
+(defvar *rectangle-skip-lines* nil
+  "If `t' -- skip empty lines, if `function' -- skip line when it returns `nil'.
+The function gets all arguments of `apply-on-rectangle' as input.")
+
 (defun 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.
+Application of the FUNCTION is affected by the `*rectangle-skip-lines*'.
 The final point after the last operation will be returned."
   (save-excursion
     (let* ((cols (rectangle--pos-cols start end))
@@ -166,7 +171,14 @@
       (goto-char startpt)
       (while
           (progn
-            (apply function startcol endcol args)
+            (when (cond
+                   ((null *rectangle-skip-lines*)
+                    t)
+                   ((functionp *rectangle-skip-lines*)
+                    (apply *rectangle-skip-lines* function start end args))
+                   (t
+                    (not (string-match-p "\\`\\s-*$" (thing-at-point 'line)))))
+              (apply function startcol endcol args))
             (setq final-point (point))
             (and (zerop (forward-line 1)) (bolp)
                  (<= (point) endpt))))