# File lib/rr/double_definitions/double_definition.rb, line 235
        def returns(*args, &implementation)
          if !args.empty? && implementation
            raise ArgumentError, "returns cannot accept both an argument and a block"
          end
          if implementation
            install_method_callback implementation
          else
            install_method_callback(lambda do |*lambda_args|
              args.first
            end)
          end
          self
        end