Include binding_set for let_expression

src/prepare-ast.js

@@ -13,6 +13,9 @@ const FIELD_MAPPINGS = {
 	let_attrset_expression: {
 		binding_set: "MERGE"
 	},
+	let_expression: {
+		binding_set: "MERGE"
+	}
 };
 
 function mergeNode(parent, child) {