PREFIX ex: <http://a.example>
PREFIX sx: <http://www.w3.org/ns/shex#>
PREFIX xsd: <http://www.w3.org/2001/XMLSchema#>

[] a sx:Schema ;
    sx:shapes (<http://a.example/S>
        <http://a.example/Sext>) .

<http://a.example/S> a sx:ShapeDecl ; sx:shapeExpr [ a sx:Shape ;
    sx:expression [ a sx:TripleConstraint ;
            sx:predicate <http://a.example/p1> ;
            sx:valueExpr <http://a.example/Sext> ] ] .

<http://a.example/Sext> a sx:ShapeDecl ; sx:shapeExpr [ a sx:ShapeExternal ] .