query: SELECT distinct ?errata ?errataLabel ?itemLabel ?item (count(?item) as ?count)
  WHERE 
  {
    hint:Query hint:optimizer "None" .
    ?errata wdt:P31 wd:Q1348305 .                  # ?errata is an errata
    filter not exists {?itemZ wdt:P2507 ?errata . } # there's no ?item pointing to the errata.
    ?errata rdfs:label ?errataLabel . filter(lang(?errataLabel)="en")
    filter(strstarts(?errataLabel,"Erratum:"))
    bind(replace(?errataLabel,"Erratum: ","") as ?itemLabel) .
    ?item rdfs:label ?itemLabel.
    filter(strlen(?itemLabel)>20)
    ?errata wdt:P1433 ?PI .
    ?item wdt:P1433 ?PI .
   # ?errata wdt:P50 ?nn .
   # ?item wdt:P50 ?nn .
   # ?errata wdt:P2093 ?nn2 .
   # ?item wdt:P2093 ?nn2 . 
  } group by ?errata ?errataLabel ?itemLabel ?item having (?count=1)