have f'_assignments_size : f'.assignments.size = n :=
  by rw [performRupCheck_preserves_assignments_size f rupHints, f_assignments_size]
n : Nat
f : DefaultFormula n
f_assignments_size : f.assignments.size = n
rupHints : Array Nat
f' : DefaultFormula n := (f.performRupCheck rupHints).fst
f'_def : f' = (f.performRupCheck rupHints).fst
⊢ List.foldl clearUnit (f.performRupCheck rupHints).fst.assignments (f.performRupCheck rupHints).snd.fst = f.assignments
n : Nat
f : DefaultFormula n
f_assignments_size : f.assignments.size = n
rupHints : Array Nat
f' : DefaultFormula n := (f.performRupCheck rupHints).fst
f'_def : f' = (f.performRupCheck rupHints).fst
f'_assignments_size : f'.assignments.size = n
⊢ List.foldl clearUnit (f.performRupCheck rupHints).fst.assignments (f.performRupCheck rupHints).snd.fst = f.assignments