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 <http://example.org/S> ] ] .