def method_missing(m, *args, &block)
      if m.to_s =~ /^with_(.+)$/
        with($1.to_sym => args.first)
        self
      else
        super
      end
    end
def method_missing(m, *args, &block)
      if m.to_s =~ /^with_(.+)$/
        with($1.to_sym => args.first)
        self
      else
        super
      end
    end
Allow users to specify fancy #with matchers.