Ġ - Group Indices by Value
This atom is monadic, which means it takes one argument.
Group the indices of a list by the corresponding values.
This returns a list of lists. Each number in
[1, 2, ..., len(z)] will appear exactly once. Each sub-list will contain indices that map to the same value; that is,
j are in the same sublist if and only if
z[i] == z[j]. The sublists are ordered by the element they correspond to.