remove duplicated code

src/function/builtin.rs

@@ -123,11 +123,8 @@ pub fn builtin_function(identifier: &str) -> Option<Function> {
         })),
         "if" => Some(Function::new(|argument| {
             let mut arguments = argument.as_fixed_len_tuple(3)?;
-            Ok(if arguments[0].as_boolean()? {
-                arguments.swap_remove(1)
-            } else {
-                arguments.swap_remove(2)
-            })
+            let result_index = if arguments[0].as_boolean()? { 1 } else { 2 };
+            Ok(arguments.swap_remove(result_index))
         })),
         "len" => Some(Function::new(|argument| {
             if let Ok(subject) = argument.as_string() {