code: function setter(prop, val) {
      var oldVal = _get(prop);
  
      if (typeof obj[prop] === 'function') {
        // Computed property setter
        obj[prop].call(tracker(prop), val);
      }
      else {
        // Simple property
        obj[prop] = val;
      }
  
      delete cache[prop];
      delete children[prop];
  
      return (oldVal !== val) && trigger('update', prop);
    }
docs: Set prop to val