unfold projection
case neg
a✝ : Formula
⊢ (projection {~a✝}).sum lengthOfFormula ≤ lengthOfFormula (~a✝)
case neg
a✝ : Formula
⊢ (let X := {~a✝};
        X.biUnion fun x => (formProjection x).toFinset).sum
      lengthOfFormula ≤
    lengthOfFormula (~a✝)