message: Fix an error message formatting in the boolean expression parser.
diff: @@ -35,7 +35,7 @@ export class BooleanExpressionEvaluator {
  case "symbol":
  const value = this._values[ast.name];
  if (value === undefined) {
  - throw Error("Undefined value '${ast.name}'");
  + throw Error(`Undefined variable '${ast.name}'`);
  }
  return value;
  case "||":
  