Double opening brackets for URI

[SArndt-TIB]

###  http://www.w3.org/ns/ssn/systems/Detection_Limit
<http://www.w3.org/ns/ssn/systems/Detection_Limit> rdf:type owl:Class ;
                                                   rdfs:subClassOf <http://www.w3.org/ns/ssn/systems/System_Property> ;
                                                   rdfs:comment "An observed value for which the probability of falsely claiming the absence of a component in a material is beta, given a probability alpha of falsely claiming its presence." ;
                                                   rdfs:isDefinedBy <<http://www.w3.org/ns/ssn/systems/DetectionLimit> ;
                                                   rdfs:label "Detection Limit"@en .

source: https://github.com/tibonto/dr/blob/82e4fb550e0e364092bc487a77598acbeda37e1c/DigitalReference.ttl#L13007