docs: function(mapping, key) {
    // Set key
    this.key = key;
  
    // Set key type
    assert(mapping[this.key], 'key \'' + key + '\' is not defined in mapping');
    assert(mapping[this.key] instanceof Types.PositiveInteger,
      'key \'' + key + '\' must be a PositiveInteger type');
    this.type = mapping[this.key];
  
    // Set covers
    this.covers = [key];
  }