Implement support for cond resolves #4

src/primitives.rs

@@ -94,7 +94,7 @@ pub fn display(args: Vec<Datum>) -> Datum {
             Number(n) => print!("{}", n),
             String(s) => print!("{}", s),
             List(v) => print!("{:?}", v),
-            _ => panic!("DISPLAY -- cannot-print {:?}", args[1]),
+            _ => panic!("DISPLAY -- cannot-print {:?}", args[i]),
         };
     }
     io::stdout().flush().ok().expect("Could not flush stdout");