have hsize : (Array.modify f.assignments l.1 addNegAssignment).size = n := by
  rw [Array.modify_preserves_size, f_readyForRupAdd.2.1]
n : Nat
f : DefaultFormula n
c : DefaultClause n
f_readyForRupAdd : f.readyForRupAdd
x✝ : Option (Literal (PosFin n))
l : PosFin n
hc : c.isUnit = some (l, false)
⊢ { clauses := f.clauses.push (some c), rupUnits := f.rupUnits, ratUnits := f.ratUnits,
      assignments := f.assignments.modify l.val addNegAssignment }.readyForRupAdd
n : Nat
f : DefaultFormula n
c : DefaultClause n
f_readyForRupAdd : f.readyForRupAdd
x✝ : Option (Literal (PosFin n))
l : PosFin n
hc : c.isUnit = some (l, false)
hsize : (f.assignments.modify l.val addNegAssignment).size = n
⊢ { clauses := f.clauses.push (some c), rupUnits := f.rupUnits, ratUnits := f.ratUnits,
      assignments := f.assignments.modify l.val addNegAssignment }.readyForRupAdd