It is slightly imprecise notation. The notation Γ′ᵢ ∈ x̅ means that the
i'th variable in the context is a first order variable y, which is in the
list x̅ of dependencies. Using Dom(Γ′ᵢ) ⊆ x̅ is not correct as it would
also include second order variables if their dependencies are in the list.
A more precise way to state this is  ∃ y s, Γ'ᵢ = (y : s) ∧ y ∈ x̅.

On Fri, Aug 22, 2025 at 4:44 PM 'Bulhwi Cha' via Metamath <
[email protected]> wrote:

> There are the definitions of FV(e) and F̲V̲(e̅::Γ′) on page 27 (Section
> 1.4.4 Well-formedness) of Mario Carneiro's PhD thesis about Metamath
> Zero[0]:
>
> FV(x) = {x}
> FV_Γ(φ) = x̅ where (φ : s x̅) ∈ Γ
> FV(f e̅) = F̲V̲(e̅::Γ′) ∪ {eᵢ | Γ′ᵢ ∈ x̅} where f(Γ′) : s x̅
>
> F̲V̲(·::·) = ∅
> F̲V̲((e̅, y)::(Γ′, x : s)) = F̲V̲(e̅::Γ′)
> F̲V̲((e̅, e′)::(Γ′, φ : s x̅)) = F̲V̲(e̅::Γ′) ∪ (FV(e′) ∖ {eᵢ | Γ′ᵢ ∈ x̅})
>
> I think {eᵢ | Dom(Γ′ᵢ) ⊆ x̅} should be substituted for {eᵢ | Γ′ᵢ ∈ x̅}
> in these definitions. Γ′ᵢ is of the form x : s, so Dom(Γ′ᵢ) is of the
> form {x}.
>
> [0] https://digama0.github.io/mm0/thesis.pdf
>
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