1 def deriv (r: Rexp, c: Char) : Rexp = r match {

     2   case NULL => NULL

     3   case EMPTY => NULL

     4   case CHAR(d) => if (c == d) EMPTY else NULL

     5   case ALT(r1, r2) => ALT(deriv(r1, c), deriv(r2, c))

     6   case SEQ(r1, r2) => 

     7     if (nullable(r1)) ALT(SEQ(deriv(r1, c), r2), deriv(r2, c))

     8     else SEQ(deriv(r1, c), r2)

     9   case STAR(r) => SEQ(deriv(r, c), STAR(r))

    10 }

    11