Every node has one and only one primary node type.
每个节点都有且只有一个主节点类型。
A primary node type defines the characteristics of the node, such as the properties and child nodes that the node is allowed to have.
主节点类型定义了节点的特征，例如允许节点拥有的属性和子节点。
Every repository must support the primary node type, nt: base. There are a number of other common node types that a repository may support.
每个仓库都必须支持主节点类型nt： base。
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