With these definitions, we can now give some conservative rules for editors in treating parts of documents corresponding to spans in a script:
It's OK for an editor to display a span if
it understands at least one of its tags.
It's OK for an editor to edit within a span if
it implements
all of its tags, and either
(a) doesn't remove any of them, or
(b) also understands all tags of its parent.
It's OK for an editor to copy a span if
it understands all the tags of the span's new parent,
no labels are moved outside their scope, and
the two environments have the same bindings for all attributes that the editor either
doesn't understand, or
knows can't be relevant anywhere in the span or its subspans.
It's OK for an editor to delete a span if
it understands all the tags of its parent.
[Less stringent rules will suffice if the document is merely to be viewed, rather than edited, using the original editor.]