1 

     2 assert(nullable(ZERO) == false)

     3 assert(nullable(ONE) == true)

     4 assert(nullable(CHAR('a')) == false)

     5 assert(nullable(ZERO | ONE) == true)

     6 assert(nullable(ZERO | CHAR('a')) == false)

     7 assert(nullable(ONE ~  ONE) == true)

     8 assert(nullable(ONE ~ CHAR('a')) == false)

     9 assert(nullable(STAR(ZERO)) == true)

    10