Add X{a..#}X test case
Alpha sequence is only letters, not all non-numbers
Do not append last item in sequences where the step passes by the end
Do not include \ in alpha sequence
Pad negative numbers after the -, not before

Ie, {-002..0} is -001 0000 0001, not 00-2 00-1 0000
Add {-10..00} test case
Add X{{a,b},}X test case
Allow negative inc's in alpha sequences
Elide out empty results, like Bash does