represent 4th variant of slots, and disallow empty primary slot names

src/atom/mod.rs

@@ -85,8 +85,9 @@ pub struct SlotName(#[get(method = "name", kind = "deref")] String);
 #[derive(Clone, Debug, PartialEq, Eq)]
 pub enum Slot {
     Wildcard,
-    Equal {
-        primary: Option<SlotName>,
+    Equal,
+    NameEqual {
+        primary: SlotName,
         sub: Option<SlotName>,
     },
     Name {
@@ -592,10 +593,11 @@ impl fmt::Display for Slot {
     fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
         match self {
             Self::Wildcard => write!(f, "*"),
-            Self::Equal { primary, sub } => {
-                if let Some(primary) = primary {
-                    write!(f, "{primary}")?;
-                }
+            Self::Equal => {
+                write!(f, "=")
+            }
+            Self::NameEqual { primary, sub } => {
+                write!(f, "{primary}")?;
 
                 if let Some(sub) = sub {
                     write!(f, "/{sub}")?;