PREFIX sx: <http://www.w3.org/ns/shex#>

[] a sx:Schema ;
  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 <http://a.example/p1> ; sx:valueExpr <http://a.example/S2> ] ] .