Correct declare_variable example

site/posts/2021-05-09-variable-declarations.md

@@ -72,9 +72,13 @@ impl Context {
 	}
 
 	fn declare_variable(&mut self, name: &str, value: Value) {
+		if self.variables.contains_key(name) {
+			panic!("cannot re-declare variable {}", name);
+		} else {
 			self.variables.insert(name.into(), value);
 		}
 	}
+}
 ```
 
 Every `eval_` function has also changed to take a reference to the current context[^1] and the main `eval` function creates a context before evaluating each statement.