layer 2: a simple string key group, or an array of keys = a nested group ordered outer→inner; layer 3: a full GroupResolver (function by + label/sort/keepTogether + its own subGroup chain)
by
layer 2: a simple string key group, or an array of keys = a nested group ordered outer→inner; layer 3: a full GroupResolver (function
by+ label/sort/keepTogether + its own subGroup chain)