code: private GinjectorBindings lowest(Set<GinjectorBindings> sources) {
      GinjectorBindings lowest = output.getGraph().getOrigin();
      while (!sources.contains(lowest)) {
        lowest = lowest.getParent();
      }
      return Preconditions.checkNotNull(lowest, "Should never make it to null");
    }
docs: Returns the member of {@code sources} closest to the origin.