drop fix

server/src/vbox.rs

@@ -129,8 +129,8 @@ impl Vbox {
     }
 
     pub fn drop(&mut self, i: usize) -> Result<&mut Vbox, Error> {
-        self.free.get(i).ok_or(format_err!("no var at index {:?}", i))?;
-        self.free.remove(i);
+        self.bound.get(i).ok_or(format_err!("no var at index {:?}", i))?;
+        self.bound.remove(i);
         // balance update
         Ok(self)
     }