Now that we've asserted the replacement slot value we can see
that, as we'd expect, the Subclass-of slot now has the value
Technical-Document instead of Document. Notice that we now have an
undo button because we have performed a side effect. The undo history
is of course multi-level. The number "1" in the button tells us how
deep the undo history is. In a fit of indecision we decide to undo
the change we have just made by clicking on the
button.