The <act>Redo</act> action allows you to recover from the last undo command.
You might want to do this if you thought that undo was going to revert
something other than what it actually did (in case you are confused
about which operations are un-doable), or if you have been backing up
through a long undo list and over-shoot your stopping point.  Any
change that is made since the undo in question will trim the redo
list.  For example if you add ten lines, then undo three of them you
could use redo to put them back, but if you move a line on the board
before performing the redo, you will lose the ability to "redo" the
three "undone" lines.