  We want to bound the symbol length of classes in ${_{2^{m-1}}Br}(F)$ which
are represented by tensor products of 5 or 6 cyclic algebras of degree $2^m$.
The main ingredients are the chain lemma for quadratic forms, a form of a
generalized Clifford invariant and Pfister's and Rost's descriptions of 12- and
14-dimensional forms in $I^3 F$.
