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

[] a sx:Schema ;
  sx:shapes (<S1> <S2>) .

<S1> a sx:ShapeDecl ; sx:shapeExpr [ a sx:Shape ;
  sx:expression <S2e> ] .

<S2> a sx:ShapeDecl ; sx:shapeExpr [ a sx:Shape ;
  sx:expression <S2e> ] .

<S2e> a sx:TripleConstraint ; sx:predicate <p1> ; sx:min 0 ; sx:max 1 ;
  sx:valueExpr <S2> .