def add_plan(plan)
      raise ArgumentError, "Invalid argument 'plan'. Expected Plan, got #{plan.class}." unless plan.is_a?(Plan)
      @plans << plan unless @associated_plans[plan.uid]
      @associated_plans[plan.uid] = plan
    end
def add_plan(plan)
      raise ArgumentError, "Invalid argument 'plan'. Expected Plan, got #{plan.class}." unless plan.is_a?(Plan)
      @plans << plan unless @associated_plans[plan.uid]
      @associated_plans[plan.uid] = plan
    end
Adds a Plan Series to this StructureSet.

 @param [Plan] plan a plan instance to be associated with this structure set