org-element: New `org-element-down' function

* contrib/lisp/org-element.el (org-element-down): New function.
* testing/lisp/test-org-element.el: Add test.

contrib/lisp/org-element.el

@@ -3859,6 +3859,17 @@ modified."
 	   ((org-before-first-heading-p) (error "No surrounding element"))
 	   (t (org-back-to-heading))))))))))
 
+(defun org-element-down ()
+  "Move to inner element."
+  (interactive)
+  (let ((element (org-element-at-point)))
+    (cond
+     ((eq (org-element-type element) 'plain-list)
+      (forward-char))
+     ((memq (org-element-type element) org-element-greater-elements)
+      (goto-char (org-element-property :contents-begin element)))
+     (t (error "No inner element")))))
+
 
 (provide 'org-element)
 ;;; org-element.el ends here

testing/lisp/test-org-element.el

@@ -312,6 +312,21 @@ Outside."
     (org-element-up)
     (should (looking-at "\\* Top"))))
 
+(ert-deftest test-org-elemnet/down-element ()
+  "Test `org-element-down' specifications."
+  ;; 1. Error when the element hasn't got a recursive type.
+  (org-test-with-temp-text "Paragraph."
+    (should-error (org-element-down)))
+  ;; 2. When at a plain-list, move to first item.
+  (org-test-with-temp-text "- Item 1\n  - Item 1.1\n  - Item 2.2"
+    (goto-line 2)
+    (org-element-down)
+    (should (looking-at " - Item 1.1")))
+  ;; 3. Otherwise, move inside the greater element.
+  (org-test-with-temp-text "#+BEGIN_CENTER\nParagraph.\n#+END_CENTER"
+    (org-element-down)
+    (should (looking-at "Paragraph"))))
+
 
 (provide 'test-org-element)
 ;;; test-org-element.el ends here