∀ {𝕜 : Type u_1} {E : Type u_2} [inst : Semifield 𝕜] [inst_1 : LinearOrder 𝕜] [IsStrictOrderedRing 𝕜]
  [inst_3 : AddCommGroup E] [inst_4 : Module 𝕜 E] {x y z : E},
  x ∈ openSegment 𝕜 y z ↔ ∃ a b, 0 < a ∧ 0 < b ∧ (a / (a + b)) • y + (b / (a + b)) • z = x