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

[] a sx:Schema ;
  sx:imports ( <3circRefS123-Icirc> ) ;
  sx:shapes (<http://a.example/S1> <http://a.example/S2> <http://a.example/S3>) .

<http://a.example/S1> a sx:ShapeDecl ; sx:shapeExpr [ a sx:Shape ; sx:expression [
    a sx:EachOf ; sx:expressions (
      [ a sx:TripleConstraint ; sx:predicate <http://a.example/p1> ]
      [ a sx:TripleConstraint ; sx:predicate <http://a.example/p2> ; sx:min 0 ; sx:max 1 ;
        sx:valueExpr <http://a.example/S2> ]
    ) ] ] .

<http://a.example/S2> a sx:ShapeDecl ; sx:shapeExpr [ a sx:Shape ; sx:expression [
    a sx:TripleConstraint ; sx:predicate <http://a.example/p3> ;
    sx:valueExpr <http://a.example/S3> ] ] .

<http://a.example/S3> a sx:ShapeDecl ; sx:shapeExpr [ a sx:Shape ; sx:expression [
    a sx:TripleConstraint ; sx:predicate <http://a.example/p4> ;
    sx:valueExpr <http://a.example/S1> ] ] .