1. A method, implemented in a kernel, for enforcing a hierarchical invocation structure between a plurality of contexts, each context having an execution priority determined by the invocation structure of the contexts, wherein there are provided lower execution priority contexts and higher execution priority contexts, the method comprising the steps of: indicating invocation of an invoked context by an invoking context, determining the execution priority of the invoking context; the kernel permitting the invocation if the execution priority of the invoked context is higher than the execution priority of the invoking context; and the kernel prohibiting the invocation if the execution priority of the invoked context is less than the execution priority of the invoking context.