Fix single prime in subscript (#1909)

* Fix single prime in subscript (a_' producing wrong result)

* Clippy

crates/typst-syntax/src/ast.rs

@@ -848,9 +848,9 @@ impl MathAttach {
             .find_map(SyntaxNode::cast)
     }
 
-    /// Extract primes if present.
+    /// Extract attached primes if present.
     pub fn primes(&self) -> Option<MathPrimes> {
-        self.0.cast_first_match()
+        self.0.children().nth(1).and_then(|n| n.cast())
     }
 }
 

tests/typ/math/opticalsize.typ

@@ -43,3 +43,7 @@ $a'_b^c$, $a_b'^c$, $a_b^c'$, $a_b'^c'^d'$
 $(a'_b')^(c'_d')$, $a'/b'$, $a_b'/c_d'$
 
 $∫'$, $∑'$, $ ∑'_S' $
+
+---
+// Test attaching primes only
+$a' = a^', a_', a_'''^''^'$