theorem liftRel_join.lem (R : α → β → Prop) {S T} {U : WSeq α → WSeq β → Prop}
    (ST : LiftRel (LiftRel R) S T)
    (HU :
      ∀ s1 s2,
        (∃ s t S T,
            s1 = append s (join S) ∧
              s2 = append t (join T) ∧ LiftRel R s t ∧ LiftRel (LiftRel R) S T) →
          U s1 s2)
    {a} (ma : a ∈ destruct (join S)) : ∃ b, b ∈ destruct (join T) ∧ LiftRelO R U a b := by
  cases' exists_results_of_mem ma with n h; clear ma; revert S T ST a
  induction' n using Nat.strongInductionOn with n IH
  intro S T ST a ra; simp [destruct_join] at ra
  exact
    let ⟨o, m, k, rs1, rs2, en⟩ := of_results_bind ra
    let ⟨p, mT, rop⟩ := Computation.exists_of_liftRel_left (liftRel_destruct ST) rs1.mem
    match o, p, rop, rs1, rs2, mT with
    | none, none, _, _, rs2, mT => by
      simp only [destruct_join]
      exact ⟨none, mem_bind mT (ret_mem _), by rw [eq_of_pure_mem rs2.mem]; trivial⟩
    | some (s, S'), some (t, T'), ⟨st, ST'⟩, _, rs2, mT => by
      simp? [destruct_append]  at rs2  says simp only [destruct_join.aux, destruct_append] at rs2
      exact
        let ⟨k1, rs3, ek⟩ := of_results_think rs2
        let ⟨o', m1, n1, rs4, rs5, ek1⟩ := of_results_bind rs3
        let ⟨p', mt, rop'⟩ := Computation.exists_of_liftRel_left (liftRel_destruct st) rs4.mem
        match o', p', rop', rs4, rs5, mt with
        | none, none, _, _, rs5', mt => by
          have : n1 < n := by
            rw [en, ek, ek1]
            apply lt_of_lt_of_le _ (Nat.le_add_right _ _)
            apply Nat.lt_succ_of_le (Nat.le_add_right _ _)
          let ⟨ob, mb, rob⟩ := IH _ this ST' rs5'
          refine' ⟨ob, _, rob⟩
          · simp (config := { unfoldPartialApp := true }) [destruct_join]
            apply mem_bind mT
            simp only [destruct_append, destruct_append.aux]
            apply think_mem
            apply mem_bind mt
            

Proof state:
case a
α : Type u
β : Type v
γ : Type w
R : α → β → Prop
U : WSeq α → WSeq β → Prop
HU :
  ∀ (s1 : WSeq α) (s2 : WSeq β),
    (∃ s t S T, s1 = append s (join S) ∧ s2 = append t (join T) ∧ LiftRel R s t ∧ LiftRel (LiftRel R) S T) → U s1 s2
n : ℕ
IH :
  ∀ (m : ℕ),
    m < n →
      ∀ {S : WSeq (WSeq α)} {T : WSeq (WSeq β)},
        LiftRel (LiftRel R) S T →
          ∀ {a : Option (α × WSeq α)}, Results (destruct (join S)) a m → ∃ b, b ∈ destruct (join T) ∧ LiftRelO R U a b
S : WSeq (WSeq α)
T : WSeq (WSeq β)
ST : LiftRel (LiftRel R) S T
a : Option (α × WSeq α)
ra : Results (Computation.bind (destruct S) destruct_join.aux) a n
o : Option (WSeq α × WSeq (WSeq α))
m k : ℕ
rs1 : Results (destruct S) o m
rs2✝ : Results (destruct_join.aux o) a k
en : n = k + m
p : Option (WSeq β × WSeq (WSeq β))
mT✝ : p ∈ destruct T
rop : LiftRelO (LiftRel R) (LiftRel (LiftRel R)) o p
s : WSeq α
S' : WSeq (WSeq α)
t : WSeq β
T' : WSeq (WSeq β)
st : LiftRel R s t
ST' : LiftRel (LiftRel R) S' T'
x✝² : Results (destruct S) (some (s, S')) m
mT : some (t, T') ∈ destruct T
rs2 : Results (Computation.think (Computation.bind (destruct s) (destruct_append.aux (join S')))) a k
k1 : ℕ
rs3 : Results (Computation.bind (destruct s) (destruct_append.aux (join S'))) a k1
ek : k = k1 + 1
o' : Option (α × WSeq α)
m1 n1 : ℕ
rs4 : Results (destruct s) o' m1
rs5 : Results (destruct_append.aux (join S') o') a n1
ek1 : k1 = n1 + m1
p' : Option (β × WSeq β)
mt✝ : p' ∈ destruct t
rop' : LiftRelO R (LiftRel R) o' p'
x✝¹ : LiftRelO R (LiftRel R) none none
x✝ : Results (destruct s) none m1
rs5' : Results (destruct_append.aux (join S') none) a n1
mt : none ∈ destruct t
this : n1 < n
ob : Option (β × WSeq β)
mb : ob ∈ destruct (join T')
rob : LiftRelO R U a ob
⊢ ob ∈ destruct_append.aux (join T') none

Next tactic:
exact mb