* Use a weighted use heuristic to disambiguate between multiple

  occurances of a component.  If the shortest path distance between a
  component P and Q in the referers graph is D, then the contribution
  of Q to the use of P is 1 / R^D, where R >= 1, typically 2.  This
  expresses that distant indirect uses are less important than nearby
  uses.

  For instance, this can disambiguate between the bootstrap GCC in
  Nixpkgs and the GCC of the final stdenv (the former has more uses,
  but they are further away),  and between the GCC of the final stdenv
  and the GCC+G77 build (the latter has very few uses).