@incollection{compref1,
   author = {Aaderaa,SO and B”rger,E},
   year = {1979},
   title = {The Horn Complexity of Boolean functions and Cook's problem},
   booktitle = {Proceedings of the 5th Scandanavian Logic Symposium},
   editor = {Mayoh,B and Jensen,F},
   publisher = {Aalborg University Press},
   address = {Aalborg},
   pages = {231-256}
}