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subtype U = A.{0,2} | C definition of subtype
A subtype definition associates a name (U) with a subset
of an existing type. The tags (A and C in the example) are
assumed to have been defined as part of a datatype definition, with
bodies which are supersets of those given in the subtype definition.
As with nametype, the same effect can be expressed by defining
the type in terms of set comprehensions, but this can be less clear and
less efficient.