code: public double getUnusedScore( final VarBindingDef binding)
      {
      // Returns the percentage of unused tuples that include this binding.
      return
        unused_.isEmpty()
        ? 0.0
  
        : (double)
          IterableUtils.countMatches(
            unused_,
            tuple -> tuple.contains( binding))
          / unused_.size();
      }
docs: Returns a measure of the "unused-ness" of the given binding.
  Returns a value between 0 and 1 (exclusive) -- a higher value means a binding is more unused.