Contracts proposal includes only member functions.

contract stringer(T) {
	T String() string
}

this means "type T shall have a method named String that returns string". A natural extension could be like:

contract sortable(S) {
	S Key int
}

sortable contract means "type S shall have a member named Key that is int". A better extension could be:

contract ordered(T) {
	T int, int8, int16, int32, int64, uint, uint8, uint16,
		uint32, uint64, uintptr, float32, float64, string
}

contract sortable2(S, T) {
	ordered(T)
	S Key T
}

sortable2 contract means "type S shall have a member named Key that is ordered". This contract could be used like:

func Sort(type S sortable2) (x []S) {
	...
	if x[i].Key < x[j].Key {
		...
	}
	...
}

Also I think this syntax is better for methods and members:

contract stringer(T) {
	T.String() string
}

contract sortable(S) {
	S.Key int
}

A further extension could be like:

type Person struct {
	Name string
	Age int
}

Say, I would like to sort a []Person w.r.t Age somewhere in my app, and w.r.t. Name at some other place in my app. I dont even want to dictate a member name "Key" to the user of my Sort() function. So I want to:

contract orderedMember(S, M) {
	ordered(S.M)
}
// or
contract orderedMember(S, M, T) {
	ordered(T)
	S.M T
}

this contract means "type S has a member M that is ordered". So that could be used like:

func Sort(type S orderedMember) (x []S){
...
	if x[i].M < x[j].M { // use M in code
...
}

var x []Person
...
Sort(M=Age) (x)
Sort(M=Name) (x)