Groups elements by applying a function, maintaining order of first appearance.
GatherBy[list, f]
GatherBy[list, {f1, f2, …}]
GatherBy[f]
GatherBy[{1, 2, 3, 4, 5}, EvenQ]
→ {{1, 3, 5}, {2, 4}}GatherBy[{-2, -1, 0, 1, 2}, Sign]
→ {{-2, -1}, {0}, {1, 2}}