sig
  type t =
      [ `Inducing_hyper of Cov_se_fat.Inducing_hyper.t
      | `Log_hetero_skedasticity of Cov_se_fat.Dim_hyper.t
      | `Log_multiscale_m05 of Cov_se_fat.Inducing_hyper.t
      | `Log_sf2
      | `Proj of Cov_se_fat.Proj_hyper.t ]
end