Module Argument.
Definition embed_var (x: nat) := atom (to_nat (x, 0)).
Definition ρ (i a b: bool) (x y: nat) :=
match i, a, b with
| false, true, _ => atom (to_nat (y, 1))
| true, false, _ => atom (to_nat (y, 1))
| _, _, false => arr (embed_var x) (atom (to_nat (y, 2)))
| _, _, true => arr (atom (to_nat (y, 3))) (embed_var x)
end.
(* simple constraints to two inequality lhs for i = false and i = true *)
Definition σ (i: bool) (p: list constraint) : term :=
fold_right (fun '((a, x), (y, b)) s => arr (ρ i a b x y) s) (atom (to_nat (0, 4))) p.
(* simple constraints to two identical inequality rhs *)
Definition τ (p: list constraint) : term :=
fold_right (fun '((a, x), (y, b)) t => arr (embed_var y) t) (atom (to_nat (0, 4))) p.
Definition src (t: term) := if t is arr s t then s else atom 0.
Definition tgt (t: term) := if t is arr s t then t else atom 0.
(* U2SemiU valuation φ from SSemiU valuation φ' *)
Definition φ (φ' : valuation) : valuation := fun x =>
match of_nat x with
| (x, 0) => substitute embed_var (φ' x)
| _ => atom x
end.
(* U2SemiU valuation ψ from SSemiU valuations φ' and ψ' *)
Definition ψ (φ' ψ' : valuation) : valuation := fun x =>
match of_nat x with
| (x, 0) => substitute embed_var (ψ' x)
| (x, 1) => substitute embed_var (φ' x)
| (x, 2) => substitute embed_var (tgt (φ' x))
| (x, 3) => substitute embed_var (src (φ' x))
| _ => atom x
end.
End Argument.

(* many-one reduction from simple semi-unification to right-uniform two-inequality semi-unification *)
Theorem reduction : SSemiU ⪯ RU2SemiU.