[Validating RDF Data] Example 68: External shape

[mistermboy]

This is the example 68 of the Validating RDF Data book:

:Product { 
    schema:productId xsd:string ;  
    schema:category EXTERNAL // :service <http://categories.org/>  
}

I wonder if the keyword "EXTERNAL" is correct because, looking at grammars, the only rule that accepts "EXTERNAL" is "shapeExprDecl":

[9]     shapeExprDecl    ::= shapeExprLabel (shapeExpression|"EXTERNAL")

Master branch grammar
Shex2.1 branch grammar
Shex.io grammar