have h := insertRatUnits_postcondition f ⟨hf.1, hf.2.1⟩ units
n : Nat
f : DefaultFormula n
hf : f.ratUnits = #[] ∧ f.assignments_invariant
units : List (Literal (PosFin n))
⊢ (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
⊢ (f.insertRatUnits units).fst.assignments_invariant