gistfile1.txt

// tuples shown instead of objects, for brevity

map output: 
[id1, "a", null]
[id1, "a", null]
[id2, "a", null]
[id2, "b", null]
[id3, "b", null]
[id4, "c", null]

what i'd like to see as a result
["a", [id1, id2] ]
["b", [id2, id3] ]
["c", [id4] ]