def with_all(*features)
      pos, neg = mangle_args(*features)
      self.class.new(Hash[@sets.select do |key, val|
        pos.subset?(key)
      end]).without_any(neg)
    end
def with_all(*features)
      pos, neg = mangle_args(*features)
      self.class.new(Hash[@sets.select do |key, val|
        pos.subset?(key)
      end]).without_any(neg)
    end
Return feature sets that include all of the given features