  We introduce the notion of b-sequence for finitely generated modules over
Noetherian rings, which characterizes long Bourbaki sequences. Our main concern
is an application of this notion to generalized Cohen-Macaulay approximation,
which we introduced lately. We will show how we can construct long Bourbaki
sequences of non-trivial type characterizing generalized Cohen-Macaualy ideals
by finding suitable b-sequences.
