rw [sdiff_singleton_is_erase] at claim
X : Finset Formula
ϕ : Formula
in_X : ϕ ∈ X
claim : lengthOf X = lengthOf (X \ {ϕ}) + lengthOfFormula ϕ
anotherClaim : insert ϕ (X \ {ϕ}) = X
⊢ lengthOf (X.erase ϕ) + lengthOfFormula ϕ = lengthOf X
X : Finset Formula
ϕ : Formula
in_X : ϕ ∈ X
claim : lengthOf X = lengthOf (X.erase ϕ) + lengthOfFormula ϕ
anotherClaim : insert ϕ (X \ {ϕ}) = X
⊢ lengthOf (X.erase ϕ) + lengthOfFormula ϕ = lengthOf X