simp only [insertRatUnits]
n : Nat
f : DefaultFormula n
hf : f.ratUnits = #[] ∧ f.assignments.size = n
units : List (Literal (PosFin n))
⊢ let assignments := (f.insertRatUnits units).fst.assignments;
  let_fun hsize := ⋯;
  let ratUnits := (f.insertRatUnits units).fst.ratUnits;
  insertUnit_invariant f.assignments ⋯ ratUnits assignments hsize
n : Nat
f : DefaultFormula n
hf : f.ratUnits = #[] ∧ f.assignments.size = n
units : List (Literal (PosFin n))
⊢ insertUnit_invariant f.assignments ⋯ (List.foldl insertUnit (f.ratUnits, f.assignments, false) units).fst
    (List.foldl insertUnit (f.ratUnits, f.assignments, false) units).2.fst ⋯