variable {α : Type}
variable (r : α → α → Prop)

/-- `SymmGen r` is the symmetric relation generated by `r`. -/
inductive SymmGen : α → α → Prop
  | rel : ∀ x y, r x y → SymmGen x y
  | symm : ∀ x y, SymmGen x y → SymmGen y x

def MyRel : Nat → Nat → Prop := SymmGen fun x y => y = x + 2

theorem preserve_add' {a : Nat} : ∀ {x y : Nat}, MyRel x y → MyRel (x + a) (y + a)
| _, _, SymmGen.rel _ _ h => SymmGen.rel _ _ (by rw [h, Nat.add_right_comm])
| _, _, SymmGen.symm _ _ h => SymmGen.symm _ _ (preserve_add' h)
termination_by structural _ _ r => r