@incollection{Heller1980,
  author    = {Heller, Klaus},
  title     = {Zum Graphembegriff},
  series = {Theoretische Probleme der deutschen Orthographie},
  editor    = {Nerius, Dieter and Scharnhorst, J{\"u}rgen},
  publisher = {Akademie-Verlag},
  address   = {Berlin},
  url       = {http://nbn-resolving.de/urn:nbn:de:bsz:mh39-83922},
  series    = {Sprache und Gesellschaft},
  number    = {Band 16},
  pages     = {74 -- 108},
  year      = {1980},
  language  = {de}
}
@incollection{Heller1979,
  author    = {Heller, Klaus},
  title     = {Zum Begriff des Graphems (Res{\"u}mee)},
  series = {Beitr{\"a}ge zu Problemen der Orthographie},
  publisher = {Akademie der Wissenschaften der DDR, Zentralinstitut f{\"u}r Sprachwissenschaft},
  address   = {Berlin},
  url       = {http://nbn-resolving.de/urn:nbn:de:bsz:mh39-83979},
  series    = {Linguistische Studien, Reihe A Arbeitsberichte},
  number    = {- 54},
  pages     = {8 -- 10},
  year      = {1979},
  language  = {de}
}