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:start <http://a.example/S1> ;
  sx:shapes (<http://a.example/S1>) .

<http://a.example/S1> a sx:ShapeDecl ; sx:shapeExpr [ a sx:Shape ;
  sx:expression
    [ a sx:TripleConstraint ; sx:predicate ex:p1 ; sx:min 0 ; sx:max -1 ;
      sx:valueExpr <http://a.example/S1> ] ] .