add eval test for let-binding with arithmetic expression

core/src/eval.rs

@@ -144,4 +144,12 @@ mod tests {
         let s = format_value(3.14);
         assert!(s.starts_with("3.14"));
     }
+
+    #[test]
+    fn eval_x_plus_5() {
+        let doc = "let x = 10\n/= x + 5";
+        let result = evaluate_document(doc);
+        assert_eq!(result.results.len(), 1);
+        assert_eq!(result.results[0].result, "15");
+    }
 }