assert(nullable(ZERO) == false)
assert(nullable(ONE) == true)
assert(nullable(CHAR('a')) == false)
assert(nullable(ZERO | ONE) == true)
assert(nullable(ZERO | CHAR('a')) == false)
assert(nullable(ONE ~  ONE) == true)
assert(nullable(ONE ~ CHAR('a')) == false)
assert(nullable(STAR(ZERO)) == true)