George M. Bergman and Ehud Hrushovski,
Linear ultrafilters,
Communications in Algebra 26 (1998) 4079-4113.
The first name of the author of reference [15] is Arnold
(not Andrew).
Reference [19] has appeared:
[19] Nicola Rodinò, Cyclic modules over
End(VD)
whose endomorphism ring is an ultrapower of D,
pp.351-357 in Abelian groups, module theory, and topology
(Padua, 1997),
Lecture Notes in Pure and Appl. Math., v.201,
Dekker, 1998.