  Let B be a C*-algebra with a maximal coaction of a locally compact group G,
and let N and H be closed normal subgroups of G with N contained in H. We show
that the process Ind_(G/H)^G which uses Mansfield's bimodule to induce
representations of the crossed product of B by G from those of the restricted
crossed product of B by (G/H) is equivalent to the two-stage induction process:
Ind_(G/N)^G composed with Ind_(G/H)^(G/N). The proof involves a calculus of
symmetric imprimitivity bimodules which relates the bimodule tensor product to
the fibred product of the underlying spaces.
