unfold projection
case And
a✝¹ a✝ : Formula
⊢ (projection {a✝¹⋀a✝}).sum lengthOfFormula ≤ lengthOfFormula (a✝¹⋀a✝)
case And
a✝¹ a✝ : Formula
⊢ (let X := {a✝¹⋀a✝};
        X.biUnion fun x => (formProjection x).toFinset).sum
      lengthOfFormula ≤
    lengthOfFormula (a✝¹⋀a✝)