simp
x : VarName
c : Char
xs : Finset VarName
h : x ∉ xs
⊢ (if h : x ∈ xs then
      let_fun this := ⋯;
      fresh { toString := x.toString ++ c.toString } c xs
    else x) ∉
    xs
x : VarName
c : Char
xs : Finset VarName
h : x ∉ xs
⊢ (if x ∈ xs then fresh { toString := x.toString ++ c.toString } c xs else x) ∉ xs