# File lib/chef_zero/solr/query/unary_operator.rb, line 29
        def matches_values?(values)
          case @operator
          when '-'
          when 'NOT'
            !operand.matches_values?(values)
          when '+'
            # TODO This operator uses relevance to eliminate other, unrelated
            # expressions.  +a OR b means "if it has b but not a, don't return it"
            raise "+ not supported yet, because it is hard."
          end
        end