@techreport{SturmfelsZiegler1991, author = {Bernd Sturmfels and G{\"u}nter M. Ziegler}, title = {Extension Spaces of Oriented Matriods.}, institution = {ZIB}, address = {Takustr. 7, 14195 Berlin}, number = {SC-91-11}, language = {eng}, year = {1991} }