multi_liner:
for x in [3..5]
for y in [3..5]
[x, y]
single_liner:
[x, y] for y in [3..5] for x in [3..5]
ok multi_liner.length is single_liner.length
ok 5 is multi_liner[2][2][1]
ok 5 is single_liner[2][2][1]