Manfred Broy Walter Dosch Helmuth Partsch Peter Pepper Martin Wirsing Existential Quantifiers in Abstract Data Types. 73-87 1979 conf/icalp/1979 ICALP db/conf/icalp/icalp79.html#BroyDPPW79 http://dx.doi.org/10.1007/3-540-09510-1_7