The preconditioners given above extend naturally to nonsymmetric
's (e.g., convection-diffusion problems), at least when the
nonsymmetric part is not too large. The nice theoretical convergence
rates can be retained provided that the coarse grid size 
is chosen
small enough (depending on the size of the nonsymmetric part of )
(see Cai and Widlund [43]).
Practical implementations (especially for parallelism) of nonsymmetric
domain decomposition methods are discussed
in [138][137].