support atoms with empty slot

src/atom/mod.rs

@@ -63,7 +63,7 @@ pub struct SlotName(#[get(method = "name")] String);
 
 #[derive(Clone, Debug, PartialEq, Eq, Get)]
 pub struct Slot {
-    slot: SlotName,
+    slot: Option<SlotName>,
     sub: Option<SlotName>,
     operator: Option<SlotOperator>,
 }