Opaque minus_rev_e.
(** * Less than predicate  - arities: (1,1) *)
Notation lt_e := minus_rev_e.

Lemma lt_correct_conv_nil v1 v2 :
unary v1 = true ->
unary v2 = true ->
sem lt_e [v1] [v2] = nil ->
length v2 <= length v1.