Drupal Version function or constant [drupal_version()]

Currently, there is no way that I have seen (at least in 4.6) to easily determine what version of Drupal you are running. Either a function or constant of (something like) drupal_version() should be added so that modules can easily tell what version of Drupal they are running on. This would allow for more flexible modules, with version-specific behaviors, and allow modules to have a longer, more easily modifiable "shelf life".