code: public static <E> Set<E> keysAbove(Counter<E> c, double countThreshold) {
      Set<E> keys = new HashSet<E>();
      for (E key : c.keySet()) {
        if (c.getCount(key) >= countThreshold) {
          keys.add(key);
        }
      }
      return (keys);
    }
docs: Returns the set of keys whose counts are at or above the given threshold.
  This set may have 0 elements but will not be null.
  
  @param c
  The Counter to examine
  @param countThreshold
  Items equal to or above this number are kept
  @return A (non-null) Set of keys whose counts are at or above the given
  threshold.