def release(builder)
      raise TypeError unless builder.is_a? @klass
      builder.reset
      @pool.push builder
      nil
    end
def release(builder)
      raise TypeError unless builder.is_a? @klass
      builder.reset
      @pool.push builder
      nil
    end
returns +builder+ back into pool *DONT* forget to lose the reference to the +builder+
 @param [Abstract]