docs: function verify(key, value) {
    var method = validator[key];
    if(typeof method === 'function') {
      // this will throw the error if verification fails
      // invert argument order as validator methods are more
      // interested in the value than the key
      value = method(value, key);
      if(value !== undefined) {
        this.set(key, value, null, true);
      }
    }
  
    return value;
  }