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/S1>
        _:S2) .

<http://a.example/S1> a sx:ShapeDecl ; sx:shapeExpr [ a sx:Shape ;
    sx:expression [ a sx:TripleConstraint ;
            sx:predicate <http://a.example/p1> ;
            sx:valueExpr _:S2 ] ] .
_:S2 a sx:ShapeDecl ; sx:shapeExpr [ a sx:Shape ;
    sx:expression [ a sx:TripleConstraint ;
      sx:predicate <http://a.example/p2> ] ] .