code: private int assertPresentAndGetIndex(String name) {
          int index = this.propertySourceList.indexOf(PropertySource.named(name));
          if (index == -1) {
              throw new IllegalArgumentException("PropertySource named '" + name + "' does not exist");
          }
          return index;
      }
docs: Assert that the named property source is present and return its index.
  
  @param name {@linkplain PropertySource#getName() name of the property source}
  to find
  @throws IllegalArgumentException if the named property source is not present