nA type definition may be a
restriction of another type's definition if it reduces permissions,
sometimes to the point of inducing obligations:
ä
\d[01]\d-\d{3}-\d{4} (a
restriction
ä (\d
)?(\d{3}-)?\d{3}-\d{4} of US p#)
nThe membership of this type, which includes
ä 207-422-6240
but not 1 337-6818
nis a (proper) subset of
the membership of the original type,
nbecause by construction
every member of the new type is a member of the original.
