@TECHREPORT{DasSti_j1990, author = {J{\"u}rgen~Dassow and Ralf~Stiebe}, title = {{On graph-theoretical closure properties of chain code picture languages}}, institution = {Technische Universit{\"a}t "`Otto von Guericke"' Magdeburg, Sektion Mathematik}, year = 1990, number = {2/90} }