Impure access

test.rs

struct Box {
  mut data : ~[float]
}

pub impl ops::Add<Box, Box> for Box
{
  pure fn add(&self, other: &Box) -> Box {
    let mut new_data : ~[float]=~[];

    let mut ctr=0;
    for self.data.each |value| {
      new_data.push(value+other.data[ctr]);
      ctr+=1;
    }

    Box{data:new_data}
  }
}

fn main() {
  let box1=Box{data:~[1.0,2.0,3.0]};
  let box2=Box{data:~[4.0,5.0,6.0]};
  let box3=box1+box2;
}