are much the same thing, and henceforth we will use the word "tag"
to mean either a tag or a property.

A tag can be a one-time tag, a propagating tag or a cancellation tag.
A one-time tag only applies to the check-in to which it is attached.  A
propagating tag applies to the check-in to which it is attached and also
to all direct descendants of that check-in.  A <i>direct descendant</i>
is a descendant through direct children.  Tags propagation does not
cross merges.  Tag propagation also stops as soon
as it encounters another check-in with the same tag.  A cancellation tag
is attached to a single check-in in order to either override a one-time
tag that was previously placed on that same check-in, or to block
tag propagation from an ancestor.

The initial checkin of every repository has two propagating tags.  In