ref:ExternalReferenceShape leaf node

@prefix rdfs: <http://www.w3.org/2000/01/rdf-schema#> .
@prefix ref: <https://brickschema.org/schema/Brick/ref#> .
@prefix sh: <http://www.w3.org/ns/shacl#> .
@prefix skos: <http://www.w3.org/2004/02/skos/core#> .
@prefix xsd: <http://www.w3.org/2001/XMLSchema#> .

ref:ExternalReferenceShape a sh:NodeShape ;
    sh:property [ sh:message "All ExternalReference must have an rdfs:label" ;
            sh:minCount 1 ;
            sh:path rdfs:label ],
        [ sh:message "All ExternalReference must have an skos:definition" ;
            sh:minCount 1 ;
            sh:path skos:definition ] ;
    sh:target [ a sh:SPARQLTarget ;
            sh:prefixes <https://brickschema.org/schema/1.4/Brick> ;
            sh:select """
    SELECT ?this WHERE {
     ?this rdfs:subClassOf+ ref:ExternalReference .
    }
    """ ] .