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