Inherited by bicg_solver_type, bicgstab2_type, bicgstab_ell_type, bicgstab_type, cg_solver_type, cgs_solver_type, fgmres_type, gcr_type, gmres_type, idr_s_type, minres_solver_type, preonly_type, qmr_solver_type, and tfqmr_solver_type.