■ Generalized A-frame (= variation) if AD = c, DC = d, same matter can be realized, c ≠ a, d ≠ b. ab = cd condition is OK. --- why? ----- Do consider by yourself. (Denken Sie nach.) cf. Generalized Hart's A-frame --- my sample a + b ≠ c + d, so, it's not an isosceles triangle. trace of node C is ⊥ AB. --- true.