Returns the root schema of the item. The context item is used if no argument is given.

• If the item is a document node, the result is the schema of its root element.
• If the item is an element node, the result is the schema of its element declaration.
• If the item is an attribute node, the result is the schema of its attribute declaration.
• If the item is a schema component, the result is the containing schema.
• If the item is an atomic value, the result is the schema containing the type of that atomic value.
• In other cases, the result is the empty sequence.