function trait(a, b){
    return function(){
      var traits = this.traits();
      var props = this.properties ? this.properties() : {};

      return get(traits, 'address.' + a)
        || get(traits, a)
        || (b ? get(traits, 'address.' + b) : null)
        || (b ? get(traits, b) : null)
        || get(props, 'address.' + a)
        || get(props, a)
        || (b ? get(props, 'address.' + b) : null)
        || (b ? get(props, b) : null);
    };
  }
Add address getters to `proto`.

@param {Function} proto