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> ] ] .