function del(key) {
  if (this.items.has(key)) {
    var currentValue = this.items.get(key);

    this.items.delete(key);
    this.inverse.items.delete(currentValue);

    // Size
    this.size = this.items.size;
    this.inverse.size = this.inverse.items.size;

    return true;
  }

  return false;
}
Method used to delete a relation.

@param  {any} key - Key.
@return {boolean}