Enforce that Ident and Lifetime are well formed
diff --git a/src/lifetime.rs b/src/lifetime.rs
index bfa59fd..44359de 100644
--- a/src/lifetime.rs
+++ b/src/lifetime.rs
@@ -8,7 +8,7 @@
 #[cfg_attr(feature = "extra-traits", derive(Debug))]
 #[cfg_attr(feature = "clone-impls", derive(Clone))]
 pub struct Lifetime {
-    pub sym: Term,
+    sym: Term,
     pub span: Span,
 }