约束可以是简单的单值，也可以是复合值或多个类型。
Constraints can be simple single values, or they can be a compound value or multiple types.
一些数据元素必须使用一组约束的值（程序员称之为 枚举类型）。
Some data elements are meant to have a set of constrained values (enumeration types as programmers would call them).
清单6是一个复杂类型的例子，其中的断言约束两个属性的值。
Listing 6 shows an example of a complex type with an assertion that constrains the values of two attributes.
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