fix: do not panic with zero-sized result

src/nodes/eval.rs

@@ -28,8 +28,8 @@ impl Nodes {
         let len = nodes.len();
         collapse_toplevel(nodes, len)?;
 
-        match nodes.pop().unwrap() {
-            Node::Number(n) => Ok(n),
+        match nodes.pop() {
+            Some(Node::Number(n)) => Ok(n),
             _ => Err(Error::CollapseFailed(self)),
         }
     }
@@ -193,6 +193,8 @@ impl Function {
 
 #[cfg(test)]
 use super::PREC;
+#[cfg(test)]
+use std::assert_matches;
 
 #[test]
 fn test() {
@@ -211,3 +213,11 @@ fn test() {
         1.9092974268256817,
     );
 }
+
+#[test]
+fn test_zerosized() {
+    assert_matches!(
+        Nodes(vec![Node::ParenthesisStart, Node::ParenthesisEnd]).evaluate(),
+        Err(Error::CollapseFailed(Nodes(v))) if v.is_empty()
+    )
+}