AdditiveMagma concept

An additive magma is a magma equipped with the addition operator.

Magma, see notes.

Corresponding functor glas::add_functor

In addition to those defined by convertible:

Name	Expression	Return type
Addition	a+b	`add_functor<A,B>::result_type`, convertible to S
AdditionAssignment	s+=b	S

Name	Expression	Precondition	Semantics	Postcondition
Addition	a+b		can actually be anything as long as it fulfills the requirements of the addition
AdditionAssignment	s+=b		equivalent to s=s+b

Closure a+b returns an object that is convertible to an element of the set S

If the type S models AdditiveMagma then the modul {S, glas::add_functor<S>} models Magma.
From the mathematical prospective AdditiveMagma could be defined without the binary operator a += b, which does not add new semantic. In order to ease the programming with concepts we require that all models of AdditiveMagma provide this operator and encourage everybody who introduces new types to add this operator consistently.