Eijiro Sumii A Complete Characterization of Observational Equivalence in Polymorphic <i>lambda</i>-Calculus with General References. 455-469 2009 CSL http://dx.doi.org/10.1007/978-3-642-04027-6_33 conf/csl/2009 db/conf/csl/csl2009.html#Sumii09