Set a de­fault value in a con­tainer.

This trait is im­pli­citly im­ple­men­ted if the con­tainer im­ple­ments Has, Get and Set, so you usu­ally won't need to ex­pli­citly im­ple­ment this.

In­ter­face

(c: Con­tainer, v: Value, k: Key) => r: Value.