TODO

   * (DONE) Add support for instance and usage lifecycle
      * (DONE) Need stop on hosting instance stops hosted instances
      * (DONE) Need to decide on semantics of stopping when instance is used by another instance (probably just stop the usages as well)
      * (DONE) Need to add stopping of usage (but not at the language level)
   * (DONE) Fix up the PDL::Interpreter::Mapping system to be a better generic hook mechanism
   * (PARTIALLY DONE) Update GraphViz visualization up to date to understand subscriptions
   * (DONE) Code review: some things are getting a little inconsistent
   * (NEED FINAL CHECK) Bring PDL::Model in line with the current meta model (System is missing, for example)
   * (DONE) Update semantics of decommisioning of instances to handle subscriptions correctly (what should they be?)
      * (DONE) Semantics: when decommissioning from a subscription just remove it from subscription
                          when decommissioning from the system remove the instance completely (including from all subscriptions)
   * (DONE) Implement ![type:id] commissioning to allow for sharing
   * (DONE) Improve install syntax to allow specifying ID for the new instance
   * (DONE) Implement query for ID
   * (DONE) Implement query for install
   * (DONE) Cleanup syntax between query and commission of usage (they are too different)
   * (DONE) Implement start/stop for usage (without this making actions that work on sets of usages are impossible to reverse)
   * (DONE) Implement resiliency checking of policies
   * Implement tracing
   * Implement parameterized capabilities (and usage of them)
   * Implement interactive mode of pdl.pl
      * Commands for listing instances, querying system, etc.
      * Commands for displaying the system
   * Bring language description in pdl.pl up to date
   * Document classes
   * Code the rest of the scenarios
   * Code the example of planning
   * Change the syntax for all of the meta operators to use []