val EVIL_urban = SEQ(STAR(STAR(CHAR('a'))), CHAR('b'))

assert(size(der('a', der('a', EVIL_urban))) == 28)
assert(size(der('a', der('a', der('a', EVIL_urban)))) == 58)

assert(size(ders("aaaaaa".toList, EVIL_urban)) == 8)