  We point out that the pure ``tree'' decays $B_c^\pm\to D^\pm_s D$ are
particularly well suited to extract the CKM angle $\gamma$ through amplitude
relations. In contrast to conceptually similar strategies using $B^\pm\to K^\pm
D$ or $B_d\to K^{\ast0} D$ decays, the advantage of the $B_c$ approach is that
the corresponding triangles have three sides of comparable length and do not
involve small amplitudes. Decays of the type $B_c^\pm\to D^\pm D$ -- the
$U$-spin counterparts of $B_c^\pm\to D^\pm_s D$ -- can be added to the
analysis, as well as channels, where the $D^\pm_s$- and $D^\pm$-mesons are
replaced by higher resonances.
