1. Create a display math inset.
2. Change the formula type to AMS Align.
3. In the first line, put A in the left box and =B in the right box. Hit
Ctrl-Enter to start the second line.
4. Ignore the left box of the second line. With the cursor in the right box,
type Ctrl-L \uncover<2->\{. The last \{ should create an inset surrounded by
braces. Type =C in that inset.
5. Hit Ctrl-Enter to start the third line, and repeat the previous step but
uncovering =D in the third overlay.Paul
