GroupBy and SubMap

int groups = 7
//divide list up into groups by % (their remainder)
Map a = (1..50).groupBy({ it % groups }) as TreeMap
assert a.keySet() == (0..<groups) as Set
assert a.keySet() == [0, 1, 2, 3, 4, 5, 6] as Set

//pull out specific keys (plus vals) from a Map
assert a.subMap(1) == [1:[1, 8, 15, 22, 29, 36, 43, 50]]
assert a.subMap([1,2]) == [1:[1, 8, 15, 22, 29, 36, 43, 50], 2:[2, 9, 16, 23, 30, 37, 44]]
assert a.subMap([1,2,99]) == [1:[1, 8, 15, 22, 29, 36, 43, 50], 2:[2, 9, 16, 23, 30, 37, 44], 99:null]
assert a.subMap([1,2,99]).findAll({k,v->!!v}) == [1:[1, 8, 15, 22, 29, 36, 43, 50], 2:[2, 9, 16, 23, 30, 37, 44]]
assert a.subMap((1..3)) == [1:[1, 8, 15, 22, 29, 36, 43, 50], 2:[2, 9, 16, 23, 30, 37, 44], 3:[3, 10, 17, 24, 31, 38, 45]]