One of the accepted techniques for developing and maintaining feature-rich applications is to treat each feature as a separate concern. However, most features are not separate concerns because they override and extend the same basic service. That is, “independent” features are coupled to one another through the system's basic service. As a result, seemingly unrelated features subtly interfere with each other when trying to override the system behaviour in different directions. The problem is how to coordinate features' access to the service's shared variables.