input: Write a function to evaluate the accuracy of a machine-learning model. The function should take as input the trained model, the test features (X_test), and the true labels (y_test), and return the accuracy score.
  assert evaluate_accuracy(clf, X_test, y_test) == 0.85
  assert evaluate_accuracy(clf, X_test_2, y_test_2) == 0.78
  assert evaluate_accuracy(clf, X_test_3, y_test_3) == 0.92
  assert evaluate_accuracy(clf, X_test_4, y_test_4) == 0.89
  assert evaluate_accuracy(None, X_test_5, y_test_5) == "Model is not defined"
output: def evaluate_accuracy(model, X_test, y_test):
      if model is None:
          return "Model is not defined"
      y_pred = model.predict(X_test)
      return accuracy_score(y_test, y_pred)