Mario Coppo Mariangiola Dezani-Ciancaglini Patrick Sallé Functional Characterization of Some Semantic Equalities inside Lambda-Calculus. 133-146 1979 conf/icalp/1979 ICALP db/conf/icalp/icalp79.html#CoppoDS79 http://dx.doi.org/10.1007/3-540-09510-1_11