let arg_spec () = [
    (arg_spec_of_symbols
       "-scm-rel"
       rel_cmd_impl_names
       rel_cmd_impl_opt
       "Backend semantics operator for SCM command relation")
    ;
    (arg_spec_of_symbols
       "-extrapolation"
       extrapolation_names
       extrapolation_opt
       "Extrapolation operator")
    ;
    (arg_spec_of_symbols
       "-extrapol-coloring"
       extrapol_coloring_names
       extrapol_coloring_opt
       "Extrapolation initial coloring")
    ;
    (arg_spec_of_symbols
       "-channel-policy"
       channel_policy_names
       channel_policy_opt
       "Channel reliability policy")
  ]