let nosib (t : 'a t) =
match t with
| [_] -> (t : [`No_siblings] t)
| _ -> failwith "foo"
dHlwZSAnYSB0ID0gJ2EgbGlzdApsZXQgbm9zaWIgKHQgOiAnYSB0KSA9CiAgbWF0Y2ggdCB3aXRoCiAgICB8IFtfXSAtPiAodCA6IFtgTm9fc2libGluZ3NdIHQpCiAgICB8IF8gICAtPiBmYWlsd2l0aCAiZm9vIgo=