@techreport {TR-1991-004,
   type = {Technical Report},
   author = {Zheng-Yang Liu},
   title = {Qualitative Reasoning about Physical Systems  with Multiple Perspectives},
   institution = {University of Oregon, Computer and Information Sciences Department},
   number = TR-1991-004,
   month = 3,
   year = 1991,
   note = {Available at \url{https://www.cs.uoregon.edu/Reports/TR-1991-004.pdf}},
}