If I have the following org document:

* Heading A
** Subheading AA
*** Subsubheading AAA
Stuff about topic AAA
*** Subsubheading AAB
Stuff about topic AAB
** Subheading AB
*** Subheading ABA
Stuff about topic ABA
*** Subheading ABB
Stuff about topic ABB
* Heading B
** Subheading BA
*** Subheading BAA
Stuff about topic BAA

How can I make it so that on startup, only the content of the first "leaf"
heading is revealed, so that the document looks like:

* Heading A
** Subheading AA
*** Subsubheading AAA
Stuff about topic AAA
*** Subsubheading AAB ...
** Subheading AB ...
* Heading B ...

Shankar Rao