fun exponent1(x,0) = 1.0
     |   exponent1(x,y) = x * exponent1(x,y-1);

     fun exponent2 x 0 = 1.0
     |   exponent2 x y = x * exponent2 x (y-1);

     exponent1(3.0,4);

     exponent2 3.0 4;