24
Permissions and obligations
n You can think of the type itself as the  set of
strings/EIIs its constraints allow.  It's helpful
to think of constraints as composed of
obligations and permissions:
 (\d )?(\d{3}-)?\d{3}-\d{4}
ä regexp definition facet for [US] 'phone number
type
n  the ? and the \d can be seen as permissions,
the - and the {3} as obligations
ä  1 337-6818 and 207-422-6240 belong to this type