have hsize : (insertRatUnits f units).1.assignments.size = n := by rw [insertRatUnits_preserves_assignments_size, hf.2.1]
n : Nat
f : DefaultFormula n
hf : f.ratUnits = #[] ∧ f.assignments_invariant
units : List (Literal (PosFin n))
h :
  let assignments := (f.insertRatUnits units).fst.assignments;
  let_fun hsize := ⋯;
  let ratUnits := (f.insertRatUnits units).fst.ratUnits;
  insertUnit_invariant f.assignments ⋯ ratUnits assignments hsize
⊢ (f.insertRatUnits units).fst.assignments_invariant
n : Nat
f : DefaultFormula n
hf : f.ratUnits = #[] ∧ f.assignments_invariant
units : List (Literal (PosFin n))
h :
  let assignments := (f.insertRatUnits units).fst.assignments;
  let_fun hsize := ⋯;
  let ratUnits := (f.insertRatUnits units).fst.ratUnits;
  insertUnit_invariant f.assignments ⋯ ratUnits assignments hsize
hsize : (f.insertRatUnits units).fst.assignments.size = n
⊢ (f.insertRatUnits units).fst.assignments_invariant