@TECHREPORT{Vaughan:2006:Binding,
  AUTHOR = {Jeffrey A. Vaughan},
  TITLE = {A Review of Three Techniques for Formally Representing
           Variable Binding},
  YEAR = {2006},
  MONTH = DEC,
  INSTITUTION = {Dept. of Computer and Information Science,
                  University of Pennsylvania},
  NUMBER = {MS-CIS-06-19}
}