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

[] a sx:Schema ;
  sx:shapes (<http://example.org/S>) .

<http://example.org/S> a sx:ShapeDecl ; sx:shapeExpr [ a sx:Shape ;
  sx:expression [
    a sx:TripleConstraint ;
    sx:predicate <http://example.org/a> ;
    sx:valueExpr [
      a sx:Shape ;
      sx:expression [
        a sx:TripleConstraint ;
        sx:predicate <http://example.org/b> ;
        sx:valueExpr <http://example.org/S>
      ]
    ]
  ] ] .