English
If x ≠ y and x,y ∈ z and x,y ∈ z' then z = z'.
Русский
Если x ≠ y и x,y ∈ z, x,y ∈ z', то z = z'.
LaTeX
$$$ \\text{If } x \\neq y \\text{ and } x,y \\in z \\text{ and } x,y \\in z' \\text{, then } z = z'. $$$
Lean4
theorem eq_of_ne_mem {x y : α} {z z' : Sym2 α} (h : x ≠ y) (h1 : x ∈ z) (h2 : y ∈ z) (h3 : x ∈ z') (h4 : y ∈ z') :
z = z' :=
((mem_and_mem_iff h).mp ⟨h1, h2⟩).trans ((mem_and_mem_iff h).mp ⟨h3, h4⟩).symm