Acceptable protocol requirements

A type X meets the AcceptableProtocol requirements if it satisfies the requirements of Protocol as well as the additional requirements listed below.

Table 1. AcceptableProtocol requirements

expression	return type	assertion/note pre/post-conditions
`X::socket`	A type that satisfies the requirements of `Destructible` (C++Std [destructible]) and `MoveConstructible` (C++Std [moveconstructible]), and that is publicly and unambiguously derived from `basic_socket<X>`.