code: @SuppressWarnings({"unchecked", "rawtypes"})
    @Override
    public Collection<InjectionPlan<?>> getChildren() {
      return (Collection) Collections.unmodifiableCollection(Arrays.asList(this.alternatives));
    }
docs: Get child elements of the injection plan tree.
  TODO: use ArrayList internally (and maybe for input, too).
  
  @return A list of injection sub-plans.