immutability

Configurations should be immutable. Compositions should preserve immutability and return new configurations. Yea or nay? I don't regard this as an important feature since most of the time clients don't need to keep around multiple configurations, but it might come in handy down the line and generally doesn't cost that much. Additionally, it is possible to provide both mutating and non-mutating versions of compositions, so one option is to have both.