1. e : EuclideanPlane
2. x : {p:Point| O_X_p}
3. y : {p:Point| O_X_p}
⊢ x + y = y + x ∈ Point
{ (DSetVars THEN Unfold `eu-add-length` 0) }
1. e : EuclideanPlane
2. x : Point
3. O_X_x
4. y : Point
5. O_X_y
⊢ (extend Ox by Xy) = (extend Oy by Xx) ∈ Point