Goal:
    X1 : sigma (fun _ : t => t)  X : lt_ (pr1 X1) (pr2 X1)
  ============================
   (forall x : t,
    {| pr1 := x; pr2 := x |} = X1 ->
    let H := CoreTactics.block in let H0 := CoreTactics.block in False)


DEBUG:Going to execute:
destruct id as [xn id]; DepElim.decompose_exists xn id;
            cbv beta iota delta [TODO projection TODO projection] in id, id';
            DepElim.decompose_exists id id'