def nullable (r: Rexp) : Boolean = r match {+ −
case NULL => false+ −
case EMPTY => true+ −
case CHAR(_) => false+ −
case ALT(r1, r2) => nullable(r1) || nullable(r2)+ −
case SEQ(r1, r2) => nullable(r1) && nullable(r2)+ −
case STAR(_) => true+ −
}+ −