remove useless import of DerefMut
@@ -27,7 +27,7 @@ use serde::{Deserialize, Serialize};
use std::collections::HashMap;
use std::ffi::OsString;
use std::fmt;
-use std::ops::{Deref, DerefMut};
+use std::ops::Deref;
use std::rc::Rc;
/// The AST of a Nickel expression.
