    # File lib/needle/interceptor.rb, line 149
        def doing( &block )
          if @doing
            raise InterceptorConfigurationError,
              "you cannot redefine 'doing' behavior"
          end

          if @with
            raise InterceptorConfigurationError,
              "cannot specify 'doing' after specifying 'with'"
          end

          if block.nil?
            raise InterceptorConfigurationError,
              "you must specify a block to 'doing'"
          end

          @doing = block
          self
        end