The type specified must be derived from the declared type or a special simple type definition called xs: error (which has no valid instances).
指定的type必须派生于声明的类型或称为xs： error的特殊简单类型定义(没有有效实例)。
Basically, the construct element with a type name matches an element whose type definition is directly or indirectly derived from the specified type.
一般而言，带有类型名的构造element匹配类型定义直接或间接来自特定类型的元素。
Determines the type definition that this derived element is based upon.
确定此派生元素所基于的类型定义。
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