aesop
X : Finset Formula
ϕ : Formula
in_X : ϕ ∈ X
claim : lengthOfSet X = lengthOfSet (X \ {ϕ}) + lengthOfFormula ϕ
anotherClaim : insert ϕ (X \ {ϕ}) = X
⊢ lengthOfSet (X.erase ϕ) + lengthOfFormula ϕ = lengthOfSet X
no goals