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> <http://a.example/S2>) .

<http://a.example/S1> a sx:ShapeDecl ; sx:shapeExpr [ a sx:ShapeOr ;
    sx:shapeExprs (
        [ a sx:ShapeNot ;
            sx:shapeExpr <http://a.example/S2>
        ]
        [ a sx:Shape ;
            sx:expression [ a sx:TripleConstraint ;
                sx:predicate <http://a.example/p2> 
            ]
        ]
    ) ] .

<http://a.example/S2> a sx:ShapeDecl ; sx:shapeExpr [ a sx:Shape ;
    sx:expression [ a sx:TripleConstraint ;
        sx:predicate <http://a.example/p3> 
    ] ] .