The paper constructs an “exotic” algebraic 2–complex over the generalized
quaternion group of order 28, with the boundary maps given by explicit matrices over
the group ring. This result depends on showing that a certain ideal of the group ring
is stably free but not free. As it is not known whether the complex constructed here
is geometrically realizable, this example is proposed as a suitable test object in
the investigation of an open problem of C T C Wall, now referred to as the
D(2)–problem.