Suppose I have an array of arrays like

[[A, B, C][1,2,3]]

I can easily make a hash (using each_with_index) where one value is
the lookup value for the other value:

1=>A
2=>B
3=>C

Now suppose I have a AoA like

[[A, B, C,][1,2,2]]

Is there a readable way to construct a hash like

1=>[A]
2=>[B, C]

?

Thanks,
-Chris