Need to return 1 if the target_geom is covered by any combination of "available" geoms.

options?

  * create a single union geom comprising of all 'available' geoms, use that to query, and then check the rate separately in the event of a match?
  * only record contiguous geoms on the availables table -- store rates as ranges on another table. Drawback: extra lookups on another table to check the rate is within bounds.
  * another better way!