code: func ComputedIf(key string, f ResourceConditionFunc) schema.CustomizeDiffFunc {
  	return func(d *schema.ResourceDiff, meta interface{}) error {
  		if f(d, meta) {
  			d.SetNewComputed(key)
  		}
  		return nil
  	}
  }
docs: // ComputedIf returns a CustomizeDiffFunc that sets the given key's new value
  // as computed if the given condition function returns true.