insertionSort [20, 7, 10, 11, 3, 87, 61, 15]
=> foldr insert [] [20, 7, 10, 11, 3, 87, 61, 15]
=> [20, 7, 10, 11, 3, 87, 61] ++ insert 15 []
=> [20, 7, 10, 11, 3, 87] ++ insert 61 [15]
=> [20, 7, 10, 11, 3] ++ insert 87 [15, 61]
=> [20, 7, 10, 11] ++ insert 3 [15, 61, 87]
=> [20, 7, 10] ++ insert 11 [3, 15, 61, 87]
=> [20, 7] ++ insert 10 [3, 11, 15, 61, 87]
=> [20] ++ insert 7 [3, 10, 11, 15, 61, 87]
=> insert 20 [3, 7, 10, 11, 15, 61, 87]
=> [3, 7, 10, 11, 15, 20, 61, 87]