article-types:letter leaf node

@prefix article-types: <http://ns.nature.com/article-types/> .
@prefix npg: <http://ns.nature.com/terms/> .
@prefix rdfs: <http://www.w3.org/2000/01/rdf-schema#> .
@prefix skos: <http://www.w3.org/2004/02/skos/core#> .
@prefix xsd: <http://www.w3.org/2001/XMLSchema#> .

article-types:letter a npg:ArticleType,
        skos:Concept ;
    npg:hasRoot article-types:research-and-reviews ;
    npg:id "letter" ;
    npg:isLeaf true ;
    npg:isRoot false ;
    npg:treeDepth 3 ;
    rdfs:isDefinedBy article-types: ;
    skos:broader article-types:article ;
    skos:definition "The :letter article-type denotes a report of original research that is shorter in format than an article."@en ;
    skos:inScheme article-types: ;
    skos:prefLabel "Letter"@en .