assert(io.mkdir("/A/B/C"))   -- creates C in /A/B, /A/B must already exist
assert(io.mkdir("./A/B/C"))  -- creates C in ./A/B, ./A/B must already exist
assert(io.mkdirs("/A/B/C"))  -- creates C in /A/B, creating A and B if necessary
assert(io.mkdirs("./A/B/C")) -- creates C in ./A/B, creating A and B if necessary