BlackoilState does not provide the used phases

During my quest to implement capillary pressure, I stumbled over the next problem with the BlackoilState class: Even though it makes use of the 'phase_usage' object at several places (e.g. in the saturations() array), there does not seem to be a way to access this if only a BlackoilState object is known. this makes this class rather less self-contained than necessary. (there is a numPhases() method, but there still no way of knowing whether the first phase represents oil, gas, or water...)