You can enter u repeatedly, reversing one step at a time, until the drawing is as it was when you
began the current editing session.

When an operation cannot be undone, the command name is displayed but no action is
performed. Operations external to the current drawing, such as plotting or writing
to a file, cannot be undone.

If you changed modes or used transparent commands during a command, their effects
are undone, along with the effects of the main command.

The U command is equivalent to entering undo 1.