Domain 1:  [a1, a2], {-3:m1, -4:m2}
Domain 2:  [a1, a3], {-3:m1, -5:m3}
Merged:    [a1, a2, a3], {-3:m1, -4:m2, -5:m3}

   [1, 2], {"m1":3, "m2":4}
 + [1, 2.5], {"m1":3, "m3":4.5}
-> [1, 2, 2.5], {"m1":3, "m2":4, "m3":4.5}

   [5, 6], {"m1":7, "m2":8}
 + [5, 6.5], {"m1":7, "m3":8.5}
-> [5, 6, 6.5], {"m1":7, "m2":8, "m3":8.5}

   [9, 10], {"m1":11, "m2":12}
 + [9, 10.5], {"m1":11, "m3":12.5}
-> [9, 10, 10.5], {"m1":11, "m2":12, "m3":12.5}

