Question about <titlealt> and @title-role

[Kristen James Eberlein <kris@eberleinconsulting.com>]

Based on the content of the stage three proposal, we ended up with the following paragraph in the spec:

"The roles of an alternative title are specified by the @title-role attribute. Multiple roles can be specified, separated by white space. An alternative title must specify at least one role."

The @title-role attribute is not required by the grammar files. Should it be? If not, what does "An alternative title must specify at least one role" mean?

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Best,
Kris