@techreport {TR-1979-009,
   type = {Technical Report},
   author = {Andrzej Proskurowski},
   title = {Centers of Two-Trees},
   institution = {University of Oregon, Computer and Information Sciences Department},
   number = TR-1979-009,
   year = 1979,
   note = {Available at \url{https://www.cs.uoregon.edu/Reports/TR-1979-009.pdf}},
}