a[3:] -> range(a, 3, undef) -> @a[3 .. 6] 'defg', expecting 'defg'
a[0:] -> range(a, 0, undef) -> @a[0 .. 6] 'abcdefg', expecting 'abcdefg'
a[-3:] -> range(a, -3, undef) -> @a[4 .. 6] 'efg', expecting 'efg'
a[:3] -> range(a, undef, 3) -> @a[0 .. 2] 'abc', expecting 'abc'
a[:0] -> range(a, undef, 0) -> @a[0 .. -1] '', expecting ''
a[:-3] -> range(a, undef, -3) -> @a[0 .. 3] 'abcd', expecting 'abcd'