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