RFC: Allow multiple traits in one interface

[KlausC]

It should be possible to define interfaces, which require more than one interface type.
Example with proposal for syntax:

@trait ProblemDescription prefix Is,IsNot
@trait Solve
@implement Tuple{IsProblemDescription,CanSolve} by solve(_2, _1, general_parameters)

[tk3369]

Love this idea. The syntax shouldn't require Tuple{} construct though?

@implement IsProblemDescription,CanSolve by solve(_2, _1, general_parameters)

[KlausC]

Agreed. Your syntax looks cleaner.
I was actually thinking of the key type of BinaryTraits.interface_map and the value type of traits_map. ( Tuple{A,B} isa Datatype). But that is independent of the user syntax.

[tk3369]

This feature is a generalization of what was proposed from #29