PREFIX rdf:     <http://www.w3.org/1999/02/22-rdf-syntax-ns#>
PREFIX rdfs:    <http://www.w3.org/2000/01/rdf-schema#>
PREFIX foaf:    <http://xmlns.com/foaf/0.1/>
PREFIX dc:      <http://purl.org/dc/elements/1.1/>
PREFIX dcterms: <http://purl.org/dc/terms/>

SELECT DISTINCT ?title
WHERE {
  ?class rdfs:subClassOf foaf:Document .
  ?doc rdf:type ?class .
  ?doc dc:title ?title .
  ?bag2 ?member2 ?doc .
  ?doc2 dcterms:references ?bag2
  OPTIONAL {
    ?class3 rdfs:subClassOf foaf:Document .
    ?doc3 rdf:type ?class3 .
    ?doc3 dcterms:references ?bag3 .
    ?bag3 ?member3 ?doc
    OPTIONAL {
      ?class4 rdfs:subClassOf foaf:Document .
      ?doc4 rdf:type ?class4 .
      ?doc4 dcterms:references ?bag4 .
      ?bag4 ?member4 ?doc3
    } FILTER (!bound(?doc4))
  } FILTER (!bound(?doc3))
}