Underbrace

    This command starts a "growing" underbrace.
The cursor sets above the underbrace symbol , so you have just to type the main part.
    To finish it, type Enter as usual.
Then the cursor sets below the underbrace symbol, so you have just to type the under part, for instance n times.
    To finish it, and then finish the symbol, type Enter as usual.
    The keyboard shortcut is Tab f.